EM? 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


AN  ESSAY 


ON 


D       E       W  , 


AND 


SEVERAL  APPEARANCES  CONNECTED 
WITH  IT. 


BY  WILLIAM  CHARLES  WELLS,  M.D., 

F.R.S.L.&E. 


PHILADELPHIA  : 
HASWELL,  BARRINGTON,  AND  HASWELL, 

293  MARKET   STREET. 

1838. 


CONTENTS. 


PAGE 

INTRODUCTION, 5 

PART    I. 

OF  THE  PHENOMENA  OP  DEW. 

SECT.   I. — Of  circumstances  which  influence  the  production  of  Dew,  .      6 

SECT.  II.— Of  the  €old  connected  with  the  formation  of  Dew,         .        .        16 

PART    II. 

OF  THE  THEORY  OF  DEW. 

Former  Theories, 27 

A  NEW  THEORY  PROPOSED. — Dew  is  the  Production  of  a  preceding  Cold 

in  the  Substances  upon  which  it  Appears,  .  .  28 
That  Cold  precedes  the  formation  of  Dew,  ascertained  by  experiment,  .  ,  29 
This  fact  applied  to  explain  several  Natural  Appearances: 

1. — The  variety  in  the  quantities  of  Dew  on  different  Bodies  exposed  to 

the  Air  during  the  same  time  of  the  night,  but  ia  different  situations,    31 
2. — The  Cold  connected  with  Dew  not  being  always  proportional  to  the 

quantity  of  that  fluid, ib. 

3. — The  production  of  Heat  by  the  formation  of  Dew,       .        .        .        ib. 
4.— The  fact  of  more  Dew  being  acquired  in  very  calm  nights,  by  sub- 
stances placed  upon  a  raised  board,  than  by  others  of  the  same  kind 
on  the  grass;  and  that  of  a  slight  agitation  of  the  atmosphere,  when 
very  pregnant  with  moisture,  increasing  the  quantity  of  Dew,        .    33 
5. — The  fact  of  Dew  never  being  formed  in  temperate  climates  upon 

the  naked  parts  of  a  living  and  healthy  human  body,      .        .        .    ib. 
6. — The  fact  of  Hygrometers  formed  of  Animal  and  Vegetable  sub- 
stances, when  exposed  to  a  clear  sky  at  night,  marking  a  degree  of 
moisture  beyond  what  is  actually  resident  in  the  Atmosphere,         .     ib. 

The  Cold  which  produces  Dew  is  itself  produced  by  the  Radiation  of  Heat 

from  those  Bodies  upon  which  Dew  is  deposited,          .        .    ib. 

The  Cold  produced  by  the  radiation  of  Heat  from  substances  upon  the  sur- 
face of  the  earth,  is  compensated  or  over-balanced  in  the  day-time  by  the 
Heat  from  the  Sun,  and  lessened  at  night  by  various  causes,  .  .  .35 

The  Cold  originating  in  the  nightly  radiation  of  Heat  from  Bodies  upon  the 
surface  of  the  Earth,  though  lessened  by  various  causes,  is  often  very  con- 
siderable,    ....  36 

Some  of  the  useful  effects  of  the  radiation  of  Heat  from  the  Earth  at  night,      38 

M363JL81 


4  CONTENTS  TO  ESSAY  ON  DEW. 

PAGE 

Observations  upon,  or  explanations  of,  the  under-mentioned  circumstances: 

1. — The  prevention,  wholly  or  in  part,  of  Cold  from  radiation,  in  sub- 
stances on  the  ground,  by  the  interposition  of  any  solid  body  between 
them  and  the  sky, 39 

2. — The  prevention,  wholly  or  in  part,  of  Cold  from  radiation,  in  sub- 
stances on  the  ground,  by  the  interposition  of  clouds,  .  .  .  ib. 

3. — The  prevention,  wholly  or  in  part,  of  cold  from  radiation,  by  fogs,       40 

4. — The  prevention,  wholly  or  in  part,  of  cold  from  radiation,  by  conduc- 
tion from  warmer  substances  in  contact  with  the  radiating  sub- 
stance,   42 

5. — The  effect  of  wind  in  compensating  the  Cold  from  radiation,  and 
sometimes  in  lessening,  and  sometimes  in  increasing,  the  production 
of  Dew, 43 

6. — The  Cold  from  radiation,  of  a  thermometer  placed  on  a  board,  being 
less  diminished  than  that  of  one  suspended  in  the  air,  .  .  .  ib. 

7. — The  hurtful  effects  of  Cold  occurring  chiefly  in  hollow  places,  ac- 
cording to  a  remark  of  Theophrastus,  .  .  .  .  .  ib. 

8. — Frost  being  less  severe  upon  hills  than  in  neighbouring  plains,  in 
calm  and  serene  nights,  .  .  .  .  ,  .  .  .44 

Reasons  assigned  for  believing  that  Air  is  actually  heated  by  the  sunbeams 
which  enter  it,  and  that  it  not  only  absorbs,  but  radiates  Heat,  .        .        .    ib. 
9. — The  leaves  of  trees  often  remaining  dry  throughout  the  night,  while 
those  of  grass  are  covered  with  Dew,     ......    49 

10. — Bright  metals  exposed  to  a  clear  sky  in  a  calm  night  being  less 
dewed  on  their  upper  surface  than  other  solid  bodies;  and  those 
metals  which  radiate  Heat  most,  being  most  attractive  of  Dew,      .    ib. 
11. — The  difference  between  black  and  white  bodies  with  respect  to 

radiation,  when  exposed  to  the  sky  at  night,  .....    52 

Whether  Dew  is  the  product  of  vapour  emitted  during  the  night  by  the 
earth  and  plants  upon  it,      i;^*r'-. :. 53 


PART    III. 

OF  SEVERAL  APPEARANCES  CONNECTED  WITH  DEW. 

1. — Of  the  greater  moisture  sometimes  observed  in  winter  mornings  upon 
the  insides  of  the  panes  of  glass  in  windows  covered  with  inside  shut- 
ters, than  upon  those  not  covered  by  them, 57 

2. — Of  the  greater  sensation  of  Cold,  which  is  sometimes  experienced  upon 
exposure  to  the  sky  in  a  clear  night,  than  is  to  be  explained  by  the  tem- 
perature of  the  Atmosphere,  «  .  .  58 

3._Of  the  effect  of  those  means  employed  by  gardeners  to  protect  tender 

plants  from  Cold  during  the  night,  which  screen  them  from  the  sky,      .    59 

4. — Of  the  effect  of  a  covering  of  snow,  or  of  other  matters,  during  still 
and  serene  nights,  in  protecting  vegetables  from  Cold,  .  .  .  .61 

5.— Of  the  putrefaction  which  has  been  supposed  to  take  place  in  animal 
substances  exposed  to  Moonshine, .62 

6.-— Of  the  formation  of  Ice  during  the  night,  in  Bengal,  when  the  tempera- 
ture of  the  air  is  above  32°,  63 

CONCLUSION, 70 


ESSAY    ON    DEW 


INTR  ODUCTION. 

I  was  led,  in  the  autumn  of  1784,  by  the  event  of  a  rude  experi- 
ment, to  think  it  probable,  that  the  formation  of  dew  is  attended  with 
the  production  of  cold.  In  1 788,  a  paper  on  hoarfrost,  by  Mr.  Patrick 
Wilson  of  Glasgow,  was  published  in  the  first  volume  of  the  Transac- 
tions of  the  Royal  Society  of  Edinburgh,  by  which  it  appeared,  that 
this  opinion  had  been  entertained  by  that  gentleman  before  it  had  oc- 
curred to  myself.  In  the  course  of  the  same  year,  Mr.  Six  of  Canter- 
bury mentioned,  in  a  paper  communicated  to  the  Royal  Society,  that, 
on  clear  and  dewy  nights,  he  always  found  the  mercury  lower  in  a  ther- 
mometer laid  upon  the  ground,  in  a  meadow  in  his  neighbourhood, 
than  it  was  in  a  similar  thermometer  suspended  in  the  air,  six  feet 
above  the  former  ;  and  that,  upon  one  night,  the  difference  amounted 
to  5°  of  Fahrenheit's  scale.  Mr.  Six,however,did  not  suppose, agree- 
ably to  the  opinion  of  Mr.  Wilson  and  myself,  that  the  cold  was 
occasioned  by  the  formation  of  dew  ;  but  imagined,  that  it  proceeded 
partly  from  the  low  temperature  of  the  air,  through  which  the  dew, 
already  formed  in  the  atmosphere,  had  descended,  and  partly  from 
the  evaporation  of  moisture  from  the  ground,  on  which  his  ther- 
mometer had  been  placed.  The  conjecture  of  Mr  Wilson,  and  the 
observations  of  Mr.  Six,  together  with  many  facts,  which  I  after- 
wards learned  in  the  course  of  reading,  strengthened  my  opinion  ; 
but  I  made  no  attempt,  before  the  autumn  of  1811,  to  ascertain  by 
experiment  if  it  were  just,  though  it  had,  in  the  mean  time,  almost 
daily  occurred  to  my  thoughts.  Happening,  in  that  season,  to  be 
in  the  country  on  a  clear  and  calm  night,  I  laid  a  thermometer  on 
grass  wet  with  dew,  and  suspended  a  second,  in  the  air,  two  feet 
above  the  other.  An  hour  afterwards,  the  thermometer  on  the 
grass  was  found  to  be  8°  lower,  by  Fahrenheit's  division,  than  the 
one  in  the  air.  Similar  results  having  been  obtained  from  several 
similar  experiments,  made  during  the  same  autumn,  I  determined 
in  the  next  spring  to  prosecute  the  subject  with  some  degree  of 
steadiness,  and  with  this  view  went  frequently  to  the  house  of  one 
of  my  friends,  who  lives  in  Surrey.  At  the  end  of  two  months, 
I  fancied  that  I  had  collected  information  worthy  of  being  published 
but  fortunately,  while  preparing  an  account  of  it,  I  met  by  accident 
with  a  small  posthumous  work  of  Mr.  Six,  printed  at  Canterbury 
in  1794,  in  which  are  related  differences  observed  on  dewy  nights, 
OCT.  1838.— Q  2 


6  ESSAY  ON  DEW. 

between  thermometers  placed  on  grass  and  others  in  the  air,  that 
are  much  greater  than  those  mentioned  in  the  paper  presented  by 
him  to  the  Royal  Society  in  1788.  In  this  work,  too,  the  cold  of 
the  grass  is  attributed,  in  agreement  with  the  opinion  of  Mr.  Wilson 
altogether  to  the  dew  deposited  upon  it.  The  value  of  my  own 
observations  appearing  to  me  now  much  diminished,  though  they 
embraced  many  points  left  untouched  by  Mr.  Six,  1  gave  up  my 
intention  of  making  them  known.  Shortly  after,  however,  upon 
considering  the  subject  more  closely,  I  began  to  suspect,  that  Mr. 
Wilson,  Mr.  Six,  and  myself,  had  all  committed  an  error,  in  regard- 
ing the  cold,  which  accompanies  dew,  as  an  effect  of  the  formation 
of  that  fluid.  I,  therefore,  resumed  my  experiments,  and  having, 
by  means  of  them,  I  think,  not  only  established  the  justness  of  my 
suspicion,  but  ascertained  the  real  cause  both  of  dew,  and  of  several 
other  natural  appearances,  which  have  hitherto  received  no  sufficient 
explanation,  I  venture  now  to  submit,  to  the  consideration  of  the 
learned,  an  account  of  some  of  my  labours,  without  regard  to  the 
order  of  time  in  which  they  were  performed,  and  of  various  con- 
clusions which  may  be  drawn  from  them,  mixed  with  facts  and 
opinions  already  published  by  others. 


PART  I. 

OP  THE  PHENOMENA  OF  DEW. 
SECTION  I. 

OP  CIRCUMSTANCES  WHICH    INFLUENCE    THE    PRODUCTION  Oi1    DEW. 

ARISTOTLE*  and  many  other  writers  have  remarked,  that  dew 
appears  only  on  calm  and  serene  nights.  The  justness  of  this 
observation,  however,  has  not  been  universally  admitted.  For 
Musschenbroekt  says,  that  dew  forms  in  Holland,  while  the  surface 
of  the  country  is  covered  with  a  low  mist ;  but,  as  he  mentions  at 
the  same  time  that  it  is  deposited  upon  all  bodies  indiscriminately, 
the  moisture,  of  which  he  speaks,  connot  properly  be  called  dew,  as 
will  be  more  distinctly  seen  hereafter.  Other  writers  of  considerable 
reputation  have  also  regarded  clearness  of  the  atmosphere,  as  not 
being  requisite  for  the  production  of  dew,  misled,  I  believe,  partly 
by  theory,  and  partly  by  observing  on  misty  mornings  copious  dews, 
which  had  been  produced  during  preceding  clear  nights.  Respect- 
ing this  point  I  can  aver,  after  much  experience,  that  I  never  knew 
dew  to  be  abundant,  except  in  serene  weather.'  In  regard  to  the 

*  Meteor.  Jjib.  I,  c.  x.  et  De  Mundo.  c.  iii. 
f  Nat.  Phil.  T.  ii.  De  Rore. 


THE  PHENOMENA  OF  DEW.  7 

necessity  of  the  air  being  still,  I  know  of  no  person  who  rejects  it, 
except  M.  Prieur,*  a  late  French  author  of  little  consideration,  and 
he  affirms,  in  opposition  to  the  most  common  observation,  that  a 
fresh  wind  is  requisite  for  the  production  of  dew. 

The  remark  of  Aristotle,  however,  is  not  to  be  received  in  its 
strictest  sense,  as  I  have  frequently  found  a  small  quantity  of  dew 
on  grass,  both  on  windy  nights,  if  the  sky  was  clear,  or  nearly  so, 
and  on  cloudy  nights,  if  there  was  no  wind.  If,  indeed,  the  clouds 
were  high,  and  the  weather  calm,  I  have  sometimes  seen  on  grass, 
though  the  sky  was  entirely  hidden,  no  very  inconsiderable  quantity 
of  dew.  Again  ;  according  to  my  observation,  entire  stillness  of 
the  atmosphere  is  so  far  from  being  necessary  for  the  formation  of 
this  fluid,  that  its  quantity  has  seemed  to  me  to  be  increased,  by  a 
very  gentle  motion  in  the  air.  Dew,  however,  has  never  been  seen 
by  me,  on  nights  both  cloudy  and  windy. 

If,  in  the  course  of  the  night,  the  weather,  from  being  calm  and 
serene,  should  become  windy  and  cloudy,  not  only  will  dew  cease 
to  form,  but  that  which  has  formed  will  either  disappear  or  di- 
minish considerably. 

In  calm  weather,  if  the  sky  be  partially  covered  with  clouds,  more 
dew  will  appear  than  if  it  were  entirely  covered,  but  less  than  if  it 
were  entirely  clear. 

Dew  probably  begins,  in  this  country,  to  appear  upon  grass,  in 
places  shaded  from  the  sun,  during  clear  and  calm  weather,  soon 
after  the  heat  of  the  atmosphere  has  declined.  My  opportunities, 
however,  for  making  such  observations  have  not  been  numerous, 
since,  while  pursuing  this  subject,  I  seldom  went  into  the  country 
till  late  in  the  afternoon  ;  but  I  have  frequently  felt  grass  moist,  in 
dry  weather,  several  hours  before  sunset.  On  the  other  hand,  I  have 
scarcely  ever  known  dew  to  be  present  in  such  quantity  upon  grass, 
as  to  exhibit  visible  drops  before  the  sun  was  very  near  the  horizon, 
or  to  be  very  copious  till  some  time  after  sunset.  It  also  continues 
to  form,  in  shaded  places,  after  sunrise  ;  but  the  interval  between 
sunrise,  and  its  ceasing  to  form,  is,  according  to  my  observation, 
which,  upon  this  point,  has  not  been  extensive,  considerably  shorter 
than  that  between  its  first  appearance  in  the  afternoon  and  sunset. 
Contrary,  however,  to  what  happens  at  sunset,  if  the  weather  be 
favourable,  more  dew  forms  a  little  before,  and,  in  shaded  places,  a 
little  after  sunrise,  than  at  any  other  time.  Musschenbroek,  there- 
fore, errs  greatly  when  he  says,  that  dew  does  not  form  after  the  sun 
has  risen.  The  preceding  observations,  on  the  early  appearance  of 
dew  in  the  afternoon,  are  to  be  restricted  to  what  happens  to  grass, 
or  other  substances  highly  attractive  of  dew  placed  on  the  ground  ; 
for  it  occurs  much  later  on  similar  substances  which  are  elevated  a 
few  feet  above  the  ground,  though  upon  these  it  continues  to  form 
as  long  after  the  rising  of  the  sun  as  upon  the  others,  if  they  be 
equally  sheltered  from  the  rays  of  that  body. 

*  Journal  de  PEcole  Polytechnique,  Tom.  ii.  409. 


8  ESSAY  ON  DEW. 

The  formation  of  dew,  after  it  has  once  commenced,  continues 
during  the  whole  night,  if  the  weather  remain  still  and  serene.  M. 
Prieur,  indeed,  of  whom  I  have  already  spoken,  asserts,  that  dew 
forms  only  in  the  evening  and  morning,  and  that  any  which  occurs 
in  the  former  season  always  disappears  in  the  course  of  the  night. 
I  can  affirm,  however,  from  long  experience,  that  grass,  after  having 
been  dewed  in  the  evening,  is  never  found  dry  until  after  sunrise, 
unless  the  weather  has,  in  the  mean  time,  changed.  Upon  one 
serene  and  still  night,  I  placed  fresh  parcels  of  wool  upon  grass  every 
hour,  and  by  weighing  each  of  them,  after  exposure  for  an  hour, 
found  that  they  had  all  attracted  dew. 

When  dew  forms  upon  a  smooth  dense  body  as  glass,  and  it  is 
only  by  means  of  such  a  body  that  the  process  can  be  accurately 
observed,  the  appearances  are  altogether  similar  to  those  which  oc- 
cur on  a  like  body  when  exposed  to  the  steam  of  water  a  little 
warmer  than  itself.  The  exposed  surface  has  first  its  lustre  dimi- 
nished by  a  slight  damp  uniformly  spread  over  it.  As  the  moisture 
increases,  it  gathers  into  irregularly  shaped  flat  drops,  which  are,  at 
first,  very  small,  but  afterwards  enlarge  and  run  into  one  another, 
forming  streamlets,  by  means  of  which  a  great  part  escapes  from  the 
body  which  had  received  it. 

During  nights  that  are  equally  clear  and  calm,  dew  often  appears 
in  very  unequal  quantities,  even  after  allowance  has  been  made  for 
any  difference  in  their  lengths.  One  great  source  of  these  differ- 
ences is  very  obvious.  For,  it  being  manifest,  whatever  theory  be 
adopted  concerning  the  immediate  cause  of  dew,  that  the  more 
replete  the  atmosphere  is  with  moisture  previously  to  the  operation 
of  that  cause,  the  more  copious  will  the  precipitation  of  water  be 
after  this  operation  has  commenced,  all  the  circumstances  which 
tend  to  increase  the  quantity  of  moisture  in  the  atmosphere,  must 
likewise  tend  to  increase  the  production  of  dew.  Thus  dew,  in 
equally  calm  and  clear  nights,  is  more  abundant  shortly  after  rain, 
than  during  a  long  tract  of  dry  weather.  It  is  more  abundant,  also, 
throughout  Europe,  with  perhaps  a  few  exceptions,  and  in  some 
parts  of  Asia  and  Africa,  during  southerly  and  westerly  winds,  than 
during  those  which  blow  from  the  north  and  the  east.  Aristotle* 
says,  that  Pontus  is  the  only  country  in  which  dew  is  more  copious 
during  a  northerly  than  during  a  southerly  wind.  But  a  similar 
fact  occurs  in  Egypt ;  for  dew  is  scarcely  ever  observed  there,  ex- 
cept while  the  Etesian  winds  prevail.  Both  cases,  however,  though 
contrary  to  the  letter,  are  consonant  with  the  spirit  of  the  rule ; 
since  the  north  wind  in  one  country  proceeds  from  the  Euxine  sea, 
and  in  the  other  from  the  Mediterranean.  Another  circumstance, 
of  the  same  kind  with  the  blowing  of  wind  from  the  south  and  the 
west,  as  showing  that  the  air  contains  much  moisture,  is  the  lessen- 
ing of  the  weight  of  the  atmosphere.  My  experience  on  this  point 
has  not,  indeed,  been  great,  as  the  falling  of  the  mercury  in  the 

*  Meteor.  Lib.  i.  c.  x. 


THE  PHENOMENA  OF  DEW.  9 

barometer  is  very  commonly  attended  with  wind  or  clouds,  both 
unfavourable  to  the  production  of  dew  ;  but  still  the  greatest  dew,  I 
have  ever  witnessed,  occurred  while  the  barometer  was  sinking.  A 
corresponding  observation  is  made  by  M.  de  Luc,  who  says,  that 
rain  may  be  foretold,  when  dew  is  uncommonly  abundant  in  relation 
to  the  climate  and  season.* 

To  the  greater  or  less  quantity  of  moisture  in  the  atmosphere,  at 
the  time  of  the  action  of  the  immediate  cause  of  dew,  are  likewise 
to  be  referred  several  other  facts  respecting  its  copiousness,  the 
explanation  of  which  is,  perhaps,  not  so  apparent  as  in  the  preced- 
ing examples. 

In  the  first  place  ;  dew  is  commonly  more  plentiful  in  spring  and 
autumn  than  in  summer  ;  the  reason  is,  that  a  greater  difference  is 
generally  found  between  the  temperatures  of  the  day  and  the  night, 
in  the  former  seasons  of  the  year  than  in  the  latter.  In  spring  this 
circumstance  is  prevented  often  from  having  a  considerable  effect, 
by  the  opposite  influence  of  northerly  and  easterly  winds ;  but,  during 
still  and  serene  nights  in  autumn,  dew  is  almost  always  highly 
abundant. 

In  the  second  place  ;  dew  is  always  very  copious  on  those  clear 
and  calm  nights  which  are  followed  by  misty  or  foggy  mornings  ; 
the  turbidness  ot  the  air  in  the  morning  showing,  that  it  must  have 
contained,  during  the  preceding  night,  a  considerable  quantity  of 
moisture. 

Thirdly  ;  I  have  observed  dew  to  be  unusually  plentiful  on  a 
clear  morning  which  had  succeeded  a  cloudy  night.  For  the  air, 
having  in  the  course  of  the  night  lost  little  or  no  moisture,  was  in 
in  the  morning  more  charged  with  watery  vapour,  than  it  would 
have  been  if  the  night  had  also  been  clear. 

Fourthly  ;  heat  of  the  atmosphere,  if  other  circumstances  are 
favourable,  which  according  to  my  experience  they  seldom  are  in 
this  country,  occasions  a  great  formation  of  dew.  For,  as  the  power 
of  the  air  to  retain  watery  vapour  in  a  pellucid  state  increases  con- 
siderably faster  while  its  temperature  is  rising,  than  in  proportion 
to  the  heat  acquired,  a  decrease  of  its  heat,  in  any  small  given 
quantity  during  the  night,  must  bring  it,  if  the  temperature  be 
high,  much  nearer  to  the  point  of  repletion,  before  it  be  acted  upon 
by  the  immediate  cause  of  dew,  than  if  the  temperature  were  low, 
We  read,  accordingly,  in  the  writings  of  those  who  have  travelled 
into  hot  climates,  of  a  copiousness  of  dew  frequently  observed  by 
them  there,  which  very  much  exceeds  what  occurs  any  time  in  this 
country.  But  even  here,  dew,  though  for  the  most  part  scanty  in 
our  hottest  season,  is  sometimes  very  abundant  during  it,  an  example 
of  which  occurred  to  me  on  the  night  common  to  the  29th  and  30th 
of  July  1813  ;  for  on  that  night,  notwithstanding  its  shortness,  more 
dew  appeared  than  has  ever  been  observed  by  me  on  any  other. 

In  the  last  place  ;  I  always  found,  when  the  clearness  and 

*  Rech.  sur  les  Mod.  de  PAtm.  §  725, 
2* 


10  ESSAY  ON  DEW. 

ness  of  the  atmosphere  were  the  same,  that  more  dew  was  formed 
between  midnight  and  sunrise,  than  between  sunset  and  midnight, 
though  the  positive  quantity  of  moisture  in  the  air  must  have  been 
less  in  the  former  than  in  the  latter  time,  in  consequence  of  a  pre- 
vious precipitation  of  part  of  it.  The  reason,  no  doubt,  is  the  cold 
of  the  atmosphere  being  greater  in  the  latter,  than  in  the  prior  part 
of  the  night. 

But  there  are  many  circumstances  influencing  the  quantity  of 
dew  which,  though  much  more  open  to  accurate  observation  than 
those  hitherto  mentioned,  are  yet  much  less  easy  to  be  understood. 

In  my  first  attempts  to  compare  the  quantities  of  dew  formed 
during  different  times,  or  in  different  situations,  I  attended  only  to 
the  appearance  which  it  made  on  bodies  having  smooth  surfaces. 
But  quickly  seeing  this  method  to  be  very  imperfect,  I  next  employed 
wool  to  collect  dew  from  the  atmosphere,  and  found  it  well  adapted 
for  my  purpose,  as  it  readily  admits  amongst  its  fibres  the  moisture 
which  forms  on  its  outer  parts,  and  retains  what  it  receives  so  firmly, 
that  I  never  but  once  had  occasion  to  suspect  that  it  suffered  any 
portion  of  what  it  had  thus  acquired  to  pass  entirely  through  it. 
The  wool  which  I  used  was  white,  moderately  fine,  and  already 
imbued  with  a  little  moisture,  from  having  been  long  exposed  to 
the  air  of  a  room  in  which  no  fire  was  kept.  I  divided  it  into 
parcels  of  10  grains  each,  and,  immediately  before  exposure,  pulled 
the  fibres  of  every  parcel  somewhat  asunder,  so  as  to  give  it  the 
form  of  a  flattened  sphere,  the  greatest  diameter  of  which  was  about 
2  inches.  As  in  doing  this  I  went  by  the  judgment  of  my  sight 
alone,  some  inequality,  in  point  of  size,  must  have  existed  among 
different  parcels,  but  none,  I  think,  sufficient  to  affect  the  accuracy 
of  my  conclusions  from  the  experiments  in  which  they  were  em- 
ployed, more  especially  as  my  conclusions  scarcely  ever  rested  upon 
single  trials. 

Previously  to  mentioning  the  results  of  any  of  my  experiments 
with  these  parcels  of  wool,  I  think  it  right  to  describe  the  place 
where  by  far  the  greater  part  of  my  observations  on  dew  were 
made.  This  was  a  garden  in  Surrey,  distant,  by  the  public  road, 
about  three  miles  from  the  bridge  over  the  Thames  at  Blackfriars,  but 
not  more  than  a  mile  and  a  quarter  from  a  densely  built  part  of  the 
suburbs  on  the  south  side  of  that  river.  The  form  of  the  garden 
was  oblong,  its  extent  nearly  half  an  acre,  and  its  surface  level. 
At  one  end  was  a  dwelling-house  of  moderate  size,  at  the  other  a 
range  of  low  buildings  ;  on  one  side  a  row  of  high  trees,  on  the 
other  a  low  fence,  dividing  it  from  another  garden.  If  this 
fence  had  been  absent,  the  garden  would  have  been  on  the  latter 
side  entirely  open.  Within  it  were  some  fruit  trees,  but,  as  it  had 
not  been  long  made,  their  size  was  small.  Towards  one  end  there 
was  a  grassplat,  in  length  62  feet,  and  nearly  16  broad,  the  herbage 
of  which  was  kept  short  by  frequent  mowing.  The  rest  of  the 
garden  was  employed  for  the  production  of  culinary  vegetables. 
All  of  these  circumstances,  however  trifling  they  may  appear,  had 
influence  on  my  experiments,  and  most  of  them,  as  will  hereafter  be 


THE  PHENOMENA  OF  DEW.  11 

seen,  must  have  rendered  the  results  less  remarkable  than  they 
would  have  been  if  they  had  occurred  on  a  wide  open  plain,  con- 
siderably distant  from  a  large  city. 

I  now  proceed  to  relate  the  influence  which  several  differences 
in  the  situation,  mechanical  state,  and  real  nature  of  bodies,  have 
upon  the  production  of  dew. 

I.  One  general  fact  relative  to  situation  is,  that  whatever  dimin- 
ishes the  view  of  the  sky,  as  seen  from  the  exposed  body,  occasions 
the  quantity  of  dew,  which  is  formed  upon  it,  to  be  less  than  would 
have  occurred  if  the  exposure  to  the  sky  had  been  complete. 

I  placed  on  several  clear  and  still  nights,  10  grains  of  wool  upon 
the  middle  of  a  painted  board,  4i  feet  long,  2  feet  wide,  and  1  inch 
thick,  elevated  4  feet  above  the  grassplat,  by  means  of  4  slender 
wooden  props  of  equal  height ;  and,  at  the  same  time,  attached, 
loosely,  10  grains  of  wool  to  the  middle  of  its  underside.  The 
two  parcels  were  consequently  only  an  inch  asunder,  and  were 
equally  exposed  to  the  action  of  the  air.  Upon  one  night,  however, 
I  found  that  the  upper  parcel  had  gained  14  grains  in  weight,  but 
the  lower  only  4.  On  a  second  night,  the  quantities  of  moisture, 
acquired  by  like  parcels  of  wool,  in  the  same  situations  as  in  the 
first  experiment,  were  19  and  6  grains  ;  on  a  third,  11  and  2  ;  on 
a  fourth,  20  and  4  ;  the  smaller  quantity  being  always  that  which 
was  gained  by  the  wool  attached  to  the  lower  side  of  the  board. 

I  bent  a  sheet  of  pasteboard  into  the  shape  of  a  house-roof,  mak- 
ing the  angle  of  flexure  90  degrees,  and  leaving  both  ends  open. 
This  was  placed  one  evening,  with  its  ridge  uppermost,  upon  the 
same  grassplat,  in  the  direction  of  the  wind,  as  well  as  this  could 
be  ascertained.  I  then  laid  10  grains  of  wool  on  the  middle  of  that 
part  of  the  grass  which  was  sheltered  by  the  roof,  and  the  same 
quantity  on  another  part  of  the  grassplat  fully  exposed  to  the  sky. 
In  the  morning,  the  sheltered  wool  was  found  to  have  increased  in 
weight  only  2  grains,  but  that  which  had  been  exposed  to  the  sky 
16  grains. 

.In  these  experiments,  the  view  of  the  sky  was  almost  entirely 
cut  off  from  the  situations  in  which  little  dew  was  formed.  In 
others,  where  it  was  less  so,  the  quantity  gained  was  greater.  Thus, 
10  grains  of  wool,  placed  upon  the  spot  of  the  grassplat,  which  was 
directly  under  the  middle  of  the  raised  board,  and  which  enjoyed, 
therefore,  a  considerable  oblique  view  of  the  sky,  acquired  during 
one  night  7,  during  a  second  9,  and  during  a  third  12  grains  of 
moisture,  while  the  quantities  gained,  during  the  same  times,  by 
equal  parcels  of  wool,  laid  upon  another  part  of  the  grassplat  which 
was  entirely  exposed  to  the  heavens,  were  10,  16,  and  20  grains. 

As  no  moisture,  falling  like  rain  from  the  atmosphere,  could  on 
a  calm  night  have  reached  the  wool  in  any  of  the  situations  where 
little  dew  was  formed,  it  may  be  thought  that  the  substances  under 
which  the  wool  was  placed,  prevented,  mechanically,  the  access  of 
that  fluid.  But  on  this  supposition  it  cannot  be  explained,  why 
some  dew  was  always  found  in  the  most  sheltered  places,  and  why 
a  considerable  quantity  occurred  upon  the  grass  under  the  middle 


12  ESSAY  ON  DEW. 

of  the  raised  board.  A  still  stronger  proof  of  the  want  of  justness 
in  this  supposition  is  afforded  by  the  following  experiment.  I 
placed,  upright,  on  the  grassplat  a  hollow  cylinder  of  baked  clay, 
the  height  of  which  was  2%  feet,  and  diameter  1  foot.  On  the  grass, 
surrounded  by  the  cylinder,  were  laid  10  grains  of  wool,  which,  in 
this  situation,  as  there  was  not  the  least  wind,  would  have  received 
as  much  rain  as  a  like  quantity  of  wool  fully  exposed  to  the  sky. 
But  the  quantity  of  moisture  obtained  by  the  wool  surrounded  by 
the  cylinder  was  only  a  little  more  than  2  grains,  while  that  ac- 
quired by  10  grains  of  fully  exposed  wool  was  16.  This  occurred 
on  the  night  during  which  the  wool  under  the  bent  pasteboard 
gained  only  2  grains  of  moisture. 

Dew,  however,  will-,  in  consequence  of  other  varieties  of  situation, 
form  in  very  different  quantities  upon  substances  of  the  same  kind, 
although  these  should  be  similarly  exposed  to  the  sky. 

In  the  first  place  ;  it  is  requisite,  for  the  most  abundant  formation 
of  dew,  that  the  substance  attracting  it  should  rest  on  a  stable  hori- 
zontal body  of  some  extent.  Thus,  upon  one  night,  while  10  grains 
of  wool,  laid  upon  the  raised  board,  increased  20  grains  in  weight, 
an  equal  quantity,  suspended  in  the  open  air,  63  feet  above  the 
ground,  increased  only  11  grains,  notwithstanding  that  it  presented 
a  greater  surface  to  the  air  than  the  other  parcel.  On  another  night, 
10  grains  of  wool  gained  on  the  raised  board  19  grains,  but  the  same 
quantity  suspended  in  the  air,  on  a  level  with  the  board,  only  13  ; 
and  on  a  third,  10  grains  of  wool  acquired  on  the  same  board  2i 
grains  of  weight,  during  the  time  in  which  other  10  grains,  hung  in 
the  air  at  the  same  height,  acquired  only  5  a  grain. 

In  the  second  place ;  the  quantities  of  dew  attracted  by  equal 
masses  of  wool,  similarly  exposed  to  the  sky,  and  resting  on  equally 
stable  and  extended  bodies,  oftentimes  vary  considerably,  in  conse- 
quence of  some  difference  in  the  other  circumstances  of  these 
bodies.  10  grains  of  wool,  for  instance,  having  been  placed  upon 
the  grassplat,  on  a  dewy  evening,  10  grains  upon  a  gravel  walk 
which  bounded  the  grassplat,  and  10  grains  upon  a  bed  of  bare 
garden  mould,  immediately  adjoining  the  gravel  walk  ;  in  the 
morning,  the  wool  on  the  grass  was  found  to  have  increased  16 
grains  in  weight,  but  that  on  the  gravel  walk  only  9,  and  that  on 
the  garden  mould  only  8.  On  another  night,  during  the  time  that 
10  grains  of  wool,  laid  upon  grass,  acquired  2%  grains  of  moisture, 
the  same  quantity  gained  only  5  a  grain  upon  the  bed  of  garden 
mould,  and  a  like  quantity,  placed  upon  the  gravel  walk,  received 
no  accession  of  weight  whatever. 

Two  objections  will  probably  be  made  against  the  accuracy  of  these, 
as  well  as  my  other  experiments  with  wool.  One  is,  that  wool 
placed  on  grass  may,  by  a  kind  of  capillary  attraction,  receive  dew 
previously  formed  on  the  grass,  in  addition  to  its  own.  To  this  I 
answer,  that  wool  in  a  china  saucer,  placed  on  the  grass,  acquired 
very  nearly  as  much  weight  as  an  equal  parcel  immediately  touch- 
ing the  grass.  The  second  objection  is,  that  a  part  of  the  increased 
weight  in  the  wool  might  arise  from  its  imbibing  moisture,  as  a 


THE  PHENOMENA  OF  DEW.  13 

^&&_        • 

hygroscopic  substance.  I  do  not  deny,  that  some  weight  was  given 
to  the  wool  in  this  way  ;  but  it  may  be  safely  affirmed,  that  this 
quantity  must  have  been  very  small.  For,  on  very  cloudy  nights, 
apparently  the  best  fitted  to  increase  the  weight  of  hygroscopic 
substances,  wool  upon  the  raised  board  would,  in  the  course  of  many 
hours,  acquire  little  or  no  weight  ;  and  in  London  I  have  never 
found  10  grains  of  wool,  exposed  to  the  air  on  the  outside  of  one  of 
my  chamber  windows,  to  increase,  during  a  whole  night,  more  than 
i  a  grain  in  weight.  When  this  weight  was  gained,  the  weather 
was  clear  and  still ;  if  the  weather  was  cloudy  and  windy,  the 
wool  received  either  less  or  no  weight.  This  window  is  so  situa- 
ted, as  to  be,  in  great  measure,  deprived  of  the  aspect  of  the  sky. 

It  being  shown  that  wool,  though  highly  attractive  of  dew,  was 
prevented,  by  the  mere  vicinity  of  a  gravel  walk,  or  a  bed  of  garden 
mould,  for  only  a  small  part  of  it  actually  touched  those  bodies, 
from  acquiring  nearly  as  much  dew  as  an  equal  parcel  laid  upon 
grass,  it  may  be  readily  inferred  that  little  was  formed  upon  them- 
selves. In  confirmation  of  this  conclusion,  I  shall  mention,  that  I 
never  saw  dew  upon  either  of  them.  Another  fact  of  the  same 
kind  is,  that,  while  returning  to  London  from  the  scene  of  my 
experiments  about  sunrise,  I  never  observed,  if  the  atmosphere  was 
clear,  the  public  road,  or  any  stone  pavement  on  the  side  of  it,  to  be 
moistened  with  dew,  though  grass  within  a  few  feet  of  it,  and 
painted  doors  and  windows  of  houses  not  far  from  it,  were  frequent- 
ly very  wet.  If,  indeed,  there  was  a  foggy  morning,  after  a  clear 
and  calm  night,  even  the  streets  of  London  would  sometimes  be 
moist,  though  they  had  been  dry  the  day  before,  and  no  rain  had  in 
the  meanwhile  fallen.  This  entire,  or  almost  entire,  freedom  of 
certain  situations  from  dew  depends,  however,  much  more  upon 
extraneous  circumstances,  than  upon  the  nature  of  the  substances 
found  there  ;  for  river  sand,  though  of  the  same  nature  with  gravel, 
when  placed  upon  the  raised  board,  or  upon  grass,  attracted  dew 
copiously. 

A  third  difference  from  situation,  in  the  quantity  of  dew  collec- 
ted by  similar  bodies,  similarly  exposed  to  the  sky,  depends  upon 
their  position  with  respect  to  the  ground.  Thus,  a  substance  placed 
several  feet  above  the  ground,  though  in  this  situation  later  dewed 
than  if  it  touched  the  earth,  would,  notwithstanding,  if  it  Jay  upon 
a  stable  body  of  some  extent,  such  as  the  raised  board  lately  men- 
tioned, acquire  more  dew  during  a  very  still  night,  than  a  similar 
substance  lying  on  grass. 

A  fourth  difference  of  this  kind  occurred  among  bodies  placed 
on  different  parts  of  the  raised  board.  For  one  that  was  placed  at 
the  leeward  end  of  it,  generally  acquired  more  dew  than  a  similar 
body  at  the  windward  extremity. 

II.  Difference  in  the  mechanical  state  of  bodies,  though  all  other 
circumstances  be  similar,  haslikewisean  effect  on  the  quantity  of  dew 
which  they  attract.  Thus,  more  dew  is  formed  upon  fine  shavings 
of  wood,  than  upon  a  thick  piece  of  the  same  substance.  It  is 


14  ESSAY  ON  DEW. 

chiefly  for  a  similar  reason,  I  believe,  that  fine  raw  silk, fine unwrought 
cotton  and  flax,  were  found  by  me  to  attract  somewhat  more  dew 
than  the  wool  I  employed,  the  fibres  of  which  were  thicker  than 
those  of  the  other  substances  just  mentioned. 

III.  Bright  metals,  in  consequence  of  some  circumstance  in  their 
constitution,  attract  dew  much  less  powerfully  than  other  bodies, 
all  of  which,  after  allowance  has  been  made  for  any  difference 
which  may  exist  in  their  mechanical  states,  seem  to  attract  dew  in 
quantities  not  very  unequal,  if  they  be  similarly  situated. 

Musschenbroek  was  the  first  who  distinctly  remarked  this 
peculiarity  of  metals ;  but  Dufay,*  I  believe,  published  it  before 
him,  referring,  at  the  same  time,  the  discovery  to  its  proper  author. 
Both  Musschenbroek  and  Dufay,  however,  made  too  large  an  infer- 
ence from  their  experiments;  for  they  asserted,  that  dew  never 
appears  on  the  upper  surface  of  bright  metals,  whereas  the  contrary 
has  since  been  observed  by  many  persons,  and  I  have  myself  known 
dew  to  form  on  gold,  silver,  copper,  tin,  platina,  iron,  steel,  zinc, 
and  lead.  Dew,  however,  when  it  does  form  upon  metals,  com- 
monly sullies  on  the  lustre  of  their  surface ;  and  even  when  it  is 
sufficiently  abundant  to  gather  into  drops,  these  are  almost  always 
small  and  distinct.  Two  other  facts  of  the  same  kind  are ;  first, 
that  the  dew,  which  has  formed  upon  a  metal,  will  often  disappear, 
while  other  substances  in  their  neighbourhood  remain  wet ;  and 
secondly,  that  a  metal,  which  has  been  purposely  moistened,  will 
often  become  dry,  though  similarly  exposed  with  bodies  which  are 
attracting  dew.  This  inaptitude  to  attract  dew  in  metals  is  commu- 
nicated to  bodies  of  a  very  different  nature  which  touch  or  are  hear 
to  them.  For  I  have  found,  that  wool  laid  upon  a  metal  will 
acquire  much  less  dew  than  an  equal  quantity  laid  upon  grass  in 
the  immediate  vicinity. 

A  large  metallic  plate,  lying  on  grass,  resists  the  formation  of  dew 
more  powerfully  than  a  very  small  one  similarly  situated.  I  con- 
clude from  various  collateral  facts,  that  a  considerable  difference  in 
the  thickness  of  two  pieces  of  metal,  exposing  equal  surfaces  to  the 
sky,  will  be  attended  with  a  similar  consequence  wherever  they  be 
placed,  though  I  have  no  observation  which  proves  this  directly. 
If,  however,  a  large  and  a  very  small  plate  be  suspended  horizon- 
tally at  the  same  height  in  the  air,  the  small  plate  will  resist  the 
formation  of  dew  more  powerfully  than  the  large. 

If  a  metal  be  closely  attached  to  a  substance  of  some  thickness 
which  attracts  dew  powerfully,  the  attraction  of  the  metal  itself  for 
dew,  instead  of  being  increased  from  this  circumstance  becomes 
diminished,  provided  the  metal  cover  the  whole  of  the  upper  surface 
of  the  other  body.  If  only  a  part  of  this  body  be  covered,  the  pro- 
duction of  dew  on  the  metal  is  forwarded  by  the  conjunction,  and 
this  somewhat  in  proportion  to  the  quantity  of  surface  in  the  lower 
body  left  uncovered.  The  justness  of  the  first  of  these  observations 

*  Mem.  de  1'Acad.  Fran.  1736. 


THE  PHENOMENA  OF  DEW.  15 

is  proved  by  the  following  experiment.  I  joined,  in  the  form  of  a 
cross,  two  pieces  of  very  light  wood,  each  4  inches  long,  a  third  of 
an  inch  in  breadth,  and  1  line  in  thickness.  To  one  side  of  this  cross  I 
fastened,  by  means  of  mucilage,  a  square  piece  of  gilt  paper,  and 
then  exposed  the  instrument  to  the  sky,  with  its  metallic  side  upper- 
most, on  a  dewy  night,  by  suspending  it,  in  a  horizontal  position, 
about  6  inches  above  the  ground.  A  few  hours  after,  the  unattached 
parts  of  the  metalled  paper  were  found  covered  with  minute  drops 
of  dew,  while  those  which  adhered  to  the  cross  were  dry. 

A  large  metallic  plate,  laid  upon  grass,  was  dewed  with  more 
difficulty  on  its  upper  surface,  than  a  similar  plate  elevated  a  few 
inches  above  the  grass  by  means  of  slender  props,  which  allowed 
the  air  to  pass  freely  under  the  metal.  But  the  case  with  respect  to 
small  pieces  was  the  reverse ;  for  I  have  often  seen  covered  with 
dew  the  metallic  sheath  of  a  small  thermometer  lying  upon  grass, 
while  the  similar  sheath  of  another  thermometer  suspended  in  the 
air  remained  dry. 

Removing  a  metal  several  times  in  the  course  of  the  night  from 
one  part  of  the  grassplat  to  another  facilitated  its  being  dewed. 
The  same  effect  was  produced  on  gilt  and  silvered  paper,  by  first 
exposing  them  to  the  sky,  for  some  time,  with  the  bare  side  upper- 
most, and  then  turning  them. 

If  a  piece  of  glass,  covered  on  one  side  with  a  metal,  be  placed 
upon  the  ground,  with  this  side  downwards,  the  upper  surface  will 
attract  dew  precisely  as  if  no  metal  were  attached  to  the  lower  surface. 

The  upper  surfaces  of  metals  are  most  readily,  and  most  copiously 
dewed  on  those  nights,  and  in  those  parts  of  the  night,  during  which 
other  substances  are  the  most  readily  and  the  most  copiously  dewed. 

If  a  metallic  plate  had  been  laid  upon  grass  before  dew  began  to 
form  anywhere,  its  lower  side,  notwithstanding,  always  became 
moist  in  the  course  of  the  night ;  and  the  same  effect  was  almost 
always  observed,  if  the  plate  had  been  placed  horizontally  in  the  air 
a  few  inches  above  the  grass.  While  the  undersides  were  thus 
moist,  the  upper  surfaces  were  very  often  dry.  If,  however,  the 
plate  was  elevated  several  feet  in  the  air,  the  condition  of  both  sides 
was  always  the  same,  whether  this  was  dry  or  moist. 

The  remarks  hitherto  made  on  the  relation  of  metals  to  dew, 
apply  to  the  class  generally  ;  but  it  is  now  to  be  mentioned,  that 
they  do  not  all  resist  the  formation  of  that  fluid  with  the  same  force. 

I  saw,  for  example,  platina  one  night  distinctly  dewed,  while  gold, 
silver,  copper,  and  tin,  though  similarly  situated,  were  entirely  dry; 
and  I  have  also  several  times  seen  these  four  metals  free  from  dew, 
while  iron,  steel,  zinc,  and  lead  were  covered  with  it. 

1  once  supposed,  in  consequence  of  the  difficulty  with  which 
metals  are  dewed,  that  they  might  in  all  circumstances  resist,  in  a 
greater  degree  than  other  bodies,  the  condensation  of  watery  vapour 
upon  their  surface  ;  and  I  afterwards  found,  that  Le  Roi*  asserts  this 

*  Mem.  de  1'Acad.  Fran.  1751. 


16  ESSAY  ON  DEW. 

to  be  the  case.  But  having  exposed  at  the  same  time,  to  the  steam 
of  warm  water,  pieces  of  glass  and  of  metal,  I  did  not  see  that 
moisture  formed  in  the  least  more  readily  upon  the  former  than 
upon  the  latter.  I  have  since  learned,  that  Saussure*  once  enter- 
tained a  similar  suspicion,  which  was  also  proved  by  an  experiment 
to  be  groundless. 

All  my  experiments,  hitherto  spoken  of,  were  made  in  the  country. 
But  Le  Roi  having  said,  that  dew  is  never  deposited  by  the  air  of 
cities,  I  determined  to  ascertain  if  his  assertion  was  just.  With 
this  view,  I  frequently  exposed,  at  night,  10  grains  of  wool  upon  a 
slight  wooden  frame,  placed  in  such  a  manner,  between  two  ridges 
of  the  top  of  my  house,  which  is  situated  in  one  of  the  most  crowded 
districts  of  London,  as  to  be  3  feet  distant  from  the  nearest  part  of 
the  roof.  The  event  was,  that,  upon  clear  and  calm  nights,  dew  was 
always  acquired  by  the  wool,  though  never  in  any  considerable 
quantity  ;  probably,  however,  more  from  the  wooden  frame  being 
nearly  surrounded  by  buildings  much  more  elevated  than  itself, 
than  from  any  particular  condition  of  the  air  in  cities.  The  forma- 
tion of  dew,  in  this  situation,  proceeded  much  less  regularly  than  in 
the  country.  For,  upon  one  evening  ten  grains  of  wool  gained  in 
it  3  grains  of  moisture  in  1  hour  and  18  minutes,  though  I  scarcely 
ever  knew  a  greater  quantity  to  be  collected  by  a  similar  parcel  of 
wool,  in  the  same  place,  during  a  whole  night.  These  experiments 
will  no  doubt  seem  to  many  superfluous,  since  dew  may  be  observed 
every  fine  evening  upon  grass  in  London.  But  as  dew  upon  grass 
is  said  by  Le  Roi  to  proceed  from  the  ground,  and  not  from  the 
atmosphere,  the  argument  derived  from  its  appearance  there,  in 
cities,  against  his  assertion  is  thus  eluded  by  him. 

The  last  subject,  which  I  shall  here  touch  upon,  is  that  of  hoar- 
frost. 

This  substance  has,  I  believe,  from  the  time  of  Aristotle,t  been 
uniformly,  and  according  to  my  observations  justly,  considered  as 
frozen  dew.  I  shall,  therefore,  frequently  refer  hereafter  to  the 
experiments  of  the  late  Mr.  Patrick  Wilson  of  Glasgow  respecting 
it,  as  if  they  had  been  actually  made  upon  that  fluid.  Indeed, 
several  of  my  experiments  upon  dew  were  only  imitations  of  some 
which  had  been  previously  made  upon  hoarfrost  by  that  ingenious 
and  most  worthy  man. 


SECTION  II. 

OF  THE  COLD  CONNECTED  WITH  THE  FORMATION  OF  DEW. 

DEW  is  often  spoken  of  as  being  cold  by  popular  writers.     Thus 
Cicero  and  Virgil  apply  to  it  the  epithet  of  "  gelidus,"  Milton  that 

*  Hygronometrie,  page  329.  f  Meteor.  Lib.  I.  c.  x. 


THE  PHENOMENA  OF  DEW.  17 

of  "  chill,"  and  Collins  that  of  "  cold."  Of  the  same  import  is  a 
passage  in  Herodotus,  in  which  it  is  said,  that  in  Egypt  the  croco- 
dile passes  a  great  part  of  the  day  on  dry  land,  but  the  whole  of  the 
night  in  the  Nile,  this  being  warmer  than  the  atmosphere,  and  the 
dew.  Among  philosophers,  however,  Mr.  Wilson  was  the  first,  I 
believe,  who  ever  suspected  the  existence  of  such  a  conjunction. 

In  my  experiments  on  the  temperature  of  bodies  moistened  with 
dew,  small  thermometers  were  employed,  (the  largest  being  only  8 
inches  long)  having  globular  bulbs,  which,  in  most  of  them,  were 
not  more  than  from  2  to  2%  lines  in  diameter.  Their  scales,  which 
were  marked  in  the  manner  of  Fahrenheit,  were  of  ivory  or  wood, 
and  were  furnished,  almost  all  of  them,  with  hinges.  They  were 
always  employed  naked,  except  I  wished  to  know  the  effect  of  cover- 
ing them  with  any  particular  substance. 

By  means  of  these  instruments  I  have  very  many  times,  during 
serene  and  still  nights,  examined  the  temperature  of  dewed  grass, 
and  have  constantly  observed  it  to  be  less  than  that  of  the  air,  any- 
where between  1  inch  and  9  feet  above  the  ground,  the  latter 
being  the  greatest  height  at  which  I  ever  marked  the  heat  of  the 
atmosphere  in  these  experiments.  I  generally,  however,  compared 
the  temperature  of  dewed  grass  with  that  of  the  air  4  feet  above  the 
ground;  and  on  nights  that  were  calm  and  clear,  very  frequently 
found  the  grass,  at  the  ordinary  place  of  my  observations,  7,  8,  or  9 
degrees  colder  than  the  air  at  that  height.  Several  times  it  was  10° 
and  11°  colder  than  the  air,  and  once  12°.  These  differences  are 
not  so  great  as  those  related  in  Mr.  Six's  posthumous  work.  But 
in  his  experiments  the  temperature  of  grass  was  compared  with  that 
of  the  air  7  feet  above  the  ground,  which,  in  clear  and  calm  nights, 
may  be  regarded  as  5  a  degree  warmer  than  the  air  at  the  height  of 
4  feet.  Besides  ;  the  most  considerable  differences,  mentioned  by 
Mr.  Six,  occurred  in  winter,  when  he  says  a  greater  degree  of  cold 
is  occasioned  by  dew,  than  at  any  other  time  ;  whereas  very  few  of 
my  experiments  on  the  temperature  of  grass  were  instituted  in  that 
season.  In  the  last  place  ;  my  experiments  were  almost  always 
made  on  very  short  grass,  while  Mr.  Six's  thermometers  were  laid 
upon  long  grass  bent,  by  strong  pressure,  towards  the  earth  ;  in 
which  state  they  marked  a  temperature  1,  2,  and  3  degrees  lower, 
than  that  shown  by  similar  thermometers  placed  upon  grass  less 
than  an  inch  in  height.  Had  it  not  been  for  these  circumstances, 
and  the  unfitness,  in  various  respects,  besides  the  shortness  of  the 
grass  for  the  production  of  a  great  cold,  of  the  common  scene  of  my 
operations,  I  believe  that,  in  consequence  of  my  thermometers  being 
much  better  adapted  to  mark  a  superficial,  or  transitory  cold,  than 
those  of  Mr.  Six,  I  should  at  some  time  have  seen  a  difference 
several  degrees  greater  than  the  greatest  ever  seen  by  that  gentle- 
man, which  was  one  of  13£°.  In  confirmation  of  this  opinion,  I 
shall  mention,  that  having,  during  a  short  visit  to  a  more  distant  part 
of  the  country,  exposed  in  the  evening  a  thermometer  upon  the 
surface  of  an  open  grass  field,  I  found  it  soon  after,  although 

OCT.  1838.—  R  3 


18  ESSAY  ON  DEW. 

the  grass  was  short,  and  the  weather  warm,  14°  lower  than  a  similar 
thermometer,  suspended  in  the  air,  4  feet  above  the  grass.  If  to 
this  quantity  be  added  £  a  degree,  on  account  of  the  difference  in 
elevation  between  our  suspended  thermometers,  the  cold,  connected 
with  dew,  observed  by  me  this  night  on  grass,  will  exceed  the 
greatest  ever  observed  by  Mr.  Six  by  1  degree. 

According  to  a  few  observations  made  by  me,  the  greater  cold- 
ness of  grass  than  that  of  the  air  begins  to  appear,  in  clear  and 
calm  weather,  in  places  sheltered  in  the  afternoon  from  the  sun, 
but  still  open  to  a  considerable  portion  of  the  sky,  soon  after  the 
heat  of  the  atmosphere  has  declined.  A  similar  coldness  continues 
upon  grass  in  still  and  serene  mornings  for  some  time  after 
the  rising  of  the  sun,  in  places  shaded  from  its  direct  light,  but 
otherwise  open  to  the  sky.  My  experiments  on  this  point  have 
also  not  been  many,  and  none  of  them  were  made  in  winter  ;  which 
I  presume  are  the  reasons  that  I  never  observed  a  cold,  from  this 
cause,  later  in  the  morning  than  an  hour  after  sunrise.  The  surface 
of  snow,  however,  was  once  in  the  depth  of  winter  observed  by 
Mr.  Wilson  of  Glasgow  to  be  considerably  colder  than  the  air,  till 
a  little  after  mid-day.* 

In  cloudy  nights,  particularly  if  there  was  wind,  the  grass  was 
never  much  colder  than  the  air.  On  such  nights  the  temperatures 
of  both  were  sometimes  the  same  ;  at  other  times  that  of  the  grass 
was  the  higher  of  the  two,  even  when  the  grass  was  wet  from  pre- 
ceding rain,  and  when,  consequently,  it  must  have  been  in  some 
measure  cooled  by  evaporation.  On  one  such  night  the  grass  was 
found  to  be  4°  colder  than  the  earth  an  inch  beneath  the  surface  of 
the  plat,  which  afforded  a  sufficient  reason  for  the  grass  itself  being 
warmer  than  the  air.  In  windy  weather,  however,  if  the  sky  was 
clear,  some  degree  of  cold,  in  addition  to  that  of  the  air,  was  always 
observed  upon  the  grass  ;  and  in  calm  weather,  very  high  clouds, 
though  sufficiently  extensive  and  dense  to  conceal  the  sky  com- 
pletely, would  yet  frequently  allow  of  the  grass  being  several 
degrees  colder  than  the  air.  I  once  observed,  upon  a  night  of  this 
kind,  a  difference  of  5°  between  the  temperature  of  those  bodies. 

If  the  night  became  cloudy,  after  having  been  very  clear,  though 
there  might  be  no  change  with  respect  to  calmness,  a  considerable 
alteration  in  the  temperature  of  the  grass  always  ensued  ;  and  this 
sometimes  very  suddenly.  Upon  one  such  night  the  grass,  after 
having  been  12°  colder  than  the  air,  became  only  2°  colder  than  it, 
the  temperature  of  the  air  being  the  same  at  both  observations.  On 
a  second  night,  grass  became  9°  warmer  in  the  space  of  an  hour  and 
a  half.  On  a  third  night,  in  less  than  45  minutes,  for  the  whole 
change  occurred  while  I  was  absent  45  minutes,  the  temperature 
of  the  grass  rose  15°,  while  that  of  the  neighbouring  air  increased 
3i°.  During  a  fourth  night,  the  temperature  of  the  grass  at  half 
past  9  o'clock  was  32°.  In  20  minutes  afterwards  it  was  found  to 

*  Paper  in  Phil.  Trans.  1781. 


THE  PHENOMENA  OF  DEW.  19 

be  39°,  the  sky  having  in  the  mean  time  become  cloudy.  At  the 
end  of  20  minutes  more,  the  sky  being  clear,  the  temperature  of  the 
grass  was  again  32°.  These  were  the  most  remarkable  of  my 
observations  on  this  subject ;  but  I  may  add  to  them,  that  I  have 
frequently  seen,  during  nights  that  were  generally  clear,  a  ther- 
mometer lying  on  the  grassplat  rise  several  degrees,  upon  the 
zenith  being  occupied  only  a  few  minutes  by  a  cloud.  On  the 
other  hand,  upon  two  nights  I  observed  a  very  great  degree  of  cold 
to  occur  on  the  ground,  in  addition  to  that  of  the  atmosphere,  during 
short  intervals  of  clearness  of  sky,  between  very  cloudy  states  of  it. 
I  did  not  speak  in  the  preceding  section  of  another  obscure  state 
of  the  atmosphere,  that  occasioned  by  fog,  or  mist,  as  the  moisture 
deposited  in  it  attaches  to  all  bodies  indiscriminately  ;  on  which 
account  I  was  unable  to  determine  whether  or  not  dew  forms  during 
its  continuance.  But,  with  respect  to  the  connexion  of  this  condi- 
tion of  the  atmosphere  with  cold,  I  have  to  remark,  that  I  have 
several  times  on  its  appearance  betwixt  day  break  and  sunrise,  found 
the  difference  between  thermometers  on  grass  and  in  the  air,  which 
had  been  considerable  during  the  night,  to  diminish  greatly.  I 
never  indeed  observed  it  to  vanish,  but  this  I  used  to  impute  to  the 
air  being  not  very  much  obscured.  I  have  now,  however,  reason  to 
doubt  the  justness  of  this  conclusion  ;  for  on  the  evening  of  the  first 
of  January  in  the  present  year,  1814,  I  found,  during  a  dense  fog, 
while  the  weather  was  very  calm,  a  thermometer  lying  on  grass, 
thickly  covered  with  hoarfrost,  9°  lower  than  another  suspended  in 
the  air,  4  feet  above  the  former.  On  the  following  evening,  when 
the  air  was  equally  calm,  but  the  fog  sufficiently  attenuated  to  allow 
me  to  see  that  the  sky  was  almost  entirely  covered  with  clouds, 
the  difference  between  two  thermometers,  similarly  placed  with  the 
former,  was  only  1°.  On  comparing  the  observations  of  these  two 
evenings,  I  conclude,  that  on  the  first  few  or  no  clouds  existed  above 
the  fog,  and  consequently  that  fog,  if  there  be  no  clouds  above  it, 
may,  in  a  very  calm  air,  admit  of  the  appearance  of  a  considerable 
degree  of  cold  at  night  upon  the  surface  of  the  earth,  in  addition  to 
that  of  the  atmosphere.  Mr.  Six,  indeed,  says,  while  speaking  of 
the  cold  connected  with  dew,  in  his  paper  in  the  Philosophical 
Transactions  for  1788,  "fogs  did  not,  as  far  as  I  could  perceive,  at 
all  impede,  but  rather  increase,  the  refrigeration."  But  this  was  a 
mistake  ;  which  in  all  probability  arose  from  his  ascribing  the  effect 
of  a  clear  night  to  an  ensuing  foggy  morning,  as  he  examined  his 
thermometers  only  in  the  daytime.  He  afterwards  discovered  his 
error  ;  for,  in  his  posthumous  work,  thick  fogs  are  ranked  among 
the  circumstances  which  always  impede,  and  sometimes  prevent 
altogether,  the  appearance  of  a  cold  upon  the  surface  of  the  earth 
greater  than  that  of  the  atmosphere.  During  a  very  dense  fog, 
Mr.  Wilson  found  no  difference,  at  night,  between  a  thermometer 
laid  upon  snow,  and  another  suspended  in  the  air.* 

*Edin.  Phil.  Trans.  I.  170. 


20  ESSAY  ON  DEW. 

When,  during  a  clear  and  still  night,  different  thermometers  were 
examined,  at  the  same  time,  which  had   been  placed  in  different 
situations,  those  which  were  situated  where  most  dew  was  formed 
were  always  found  to  be  the  lowest.     Thus,  upon  one  such  night,  I 
found  a  thermometer  placed  upon  a  little  wool,  lying  upon  the  middle 
of  the  upper  side  of  the  raised  board,  to  be  9°  lower  than  another 
thermometer,  in  contact  with  an  equal  quantity  of  wool  attached  to  the 
middle  of  the  underside  of  the  board.     On  two  other  nights  the  dif- 
ference between  two  thermometers  in  the  same  situations  was  8°.     I 
found  also,  on  two  other  serene  and  calm  nights,  a  spot  of  grass  co- 
vered by  the  pasteboard  roof,  and  another  spot  surrounded  by  the 
earthen  cylinder,  to  be  both  10°  warmer  than  neighbouring  grass  fully 
exposed  to  the  sky.    Thinking  it  possiblethat  the  cylinder,  which  had 
been  exposed  to  the  sun  the  preceding  clay,  might  still  possess  some 
of  the  heat  which  it  had  then  imbibed,  1  placed  near  to  it,  on  another 
night,  a  cylinder  made   of  very  thin    pasteboard  ;    but  this  was 
equally  efficacious  with  the   earthen  one  in  preventing  cold  from 
occurring  on  grass.     When  the  exposure  was  greater  than  in  the 
preceding  examples,  and  more  dew  was  in  consequence  formed,  the 
cold  was  also  greater,  but  still  less  than  where  the  exposure  was 
complete.     For  instance,  upon  the  night  during  which  10  grains  of 
wool  placed  upon  the  middle  of  the  grass,  which  was  sheltered  by 
the  raised  board,  had  gained   7  grains,  and   the  same  quantity  on 
grass  fully  exposed  to  the  sky  had  gained  10  grains,  the  difference 
between  the  temperatures  of  the  two  portions  of  grass  was  only  25°. 
The  same  correspondence  was  observed,  when  the  differences  in 
the  quantity  of  dew  did  not  depend,  as  in  the  preceding  instances, 
upon  any  diversity  of  exposure  to  the  sky.     Thus,  the  mercury  in 
a  thermometer  placed  upon  wool,  lying  on  the  raised   board,  was 
found  to  be  at  the  44th  degree,  while  that  in   another,  pendent  in 
the  air,  at  the  same  height  from  the  ground,  and  wrapped  in  wool, 
was  at  the  48th.     Wool,  also,  on  the  raised  board,*  was  commonly 
a  little  colder  than  the  same  substance  on  grass,  when  the  night  was 
very  still  ;  and  the  leeward  end  of  that  board  was  generally  colder 
than  the  windward  extremity. 

But  the  most  remarkable  examples  of  this  kind  were  exhibited 
by  the  gravel  walk  and  the  bare  garden  mould.  In  still  and  serene 
nights  the  surfaces  of  these  bodies  were  always  warmer  than  the 
neighbouring  grass,  and  frequently  warmer  than  the  air.  On  one 
night  of  this  description,  I  observed  2^  hours  after  sunset,  the  sur- 
face of  the  gravel  walk  to  be  16^°,  and  that  of  the  garden  mould  to 
be  12i°,  warmer  than  grass  very  near  to  them,  and  similarly  exposed 

*  The  greater  cold  of  the  raised  board  in  ray  experiments,  most  probably  de- 
pended" on  the  grass  being  very  short;  since  Mr.  Wilson  found,  that  snow  on  the 
ground  was  colder  than  the  same  body  on  a  raised  board.  If  1,2,  orw3  degrees 
were  added  to  the  cold  of  the  grass  at  my  place  of  observation,  agreeably  to  the 
difference  found  by  Mr.  Six,  between  the  temperatures  of  long  and  short  grass  in 
dewy  nights,  the  cold  on  my  raised  board  would,  upon  such  nights,  have  been 
always  less  than  that  of  the  grassplat. 


THE  PHENOMENA  OF  DEW.  21 

to  the  heavens.-  As  the  night  proceeded,  clouds  formed  and  accu- 
mulated ;  in  consequence  of  which  the  difference  at  sunrise,  between 
the  temperatures  of  the  grass  and  the  gravel  walk,  was  only  6°,  and 
between  those  of  the  grass  and  the  mould  only  4°,  the  temperature 
of  the  grass  having  in  the  mean  time  increased  considerably,  while 
that  of  the  other  bodies  had  decreased  a  little.  At  another  time, 
shortly  before  sunrise,  a  very  clear  morning  having  succeeded  a 
cloudy  night,  I  found  the  gravel  walk  to  be  10°  and  the  garden  bed  to 
be  9°  warmer  than  neighbouring  grass,  which  was  8°  colder  than  the 
air.  Both  of  these  examples  occurred  in  summer,  and  I  believe 
that  such  considerable  differences  will  occur  in  that  season  only. 
It  was  on  the  first  of  these  two  nights,  that  10  grains  of  wool  gained 
only  $  a  grain  of  moisture  on  the  mould,  and  that  the  same  quantity 
gained  no  weight  on  the  gravel  walk.  That  the  unfitness  of  the 
gravel  walk,  however,  to  become  cold,  like  its  unfitness  to  attract 
dew,  arose  from  its  situation,  and  not  from  the  nature  of  the  sub- 
stance of  which  it  was  made,  is  proved  by  this  circumstance,  that 
river  sand,  placed  on  the  raised  board,  was  on  four  different  nights, 
none  of  them  highly  favourable  for  the  production  of  cold  7,  7,  8 
and  8^  degrees  colder  than  the  air  at  the  same  height. 

It  may  be  added  here,  that  I  have  always  found,  on  dewy  nights, 
the  temperature  of  the  earth,  \  an  inch  or  an  inch  beneath  its 
surface,  much  warmer  than  the  grass  upon  it.  On  five  such  nights 
the  differences  were  from  12  to  16  degrees.  The  earth,  at  the  above 
mentioned  depth,  was  also  almost  constantly  warmer  on  dewy 
nights  than  the  air;  sometimes  it  was  considerably  so,  for  I  once 
observed  it  to  be  10°  warmer,  at  another  time  9°,  and  at  a  third  7i°. 
An  exception  will  no  doubt  occur  if  very  mild  weather  should 
follow  a  long  frost ;  but  of  this  I  have  had  no  experience. 

In  the  experiments  upon  my  housetop  in  London,  I  always  found, 
during  clear  and  calm  nights,  wool  lying  on  the  wooden  frame  to  be 
colder  than  the  air,  at  the  same  height  ;  but  the  difference  was 
seldom  more  than  3°.  On  the  evening,  however,  during  which 
dew  formed  there  more  copiously  than  usual,  the  difference  was  5°. 
That  the  smallness  of  these  differences  was  not  wholly  occasioned 
by  any  thing  special  in  the  air  of  cities  was  afterwards  proved,  by 
my  finding  others  much  greater,  in  a  garden  nearly  in  the  middle 
of  London,  from  which  almost  the  whole  of  the  sky  was  visible. 

Metals,  likewise,  furnish  proofs  of  the  connexion  of  dew  with  a 
cold  in  the  substance  on  which  it  forms,  superior  to  that  of  the 
neighbouring  atmosphere.  My  observations,  however,  on  the 
temperature  of  metals,  when  exposed  to  the  sky  on  dewy  nights 
were  less  numerous  than  those  on  several  other  subjects  treated  in 
this  Essay,  by  reason  of  the  less  frequent  opportunity  I  enjoyed  of 
making  them  ;  and  many  of  those  which  I  did  make  were  after- 
wards found  by  me  to  have  been  improperly  conducted.  I  thought, 
for  instance,  for  some  time,  that  the  temperature  of  a  metal,  on  a 
dewy  night,  might  easily  be  learned  in  the  way  in  which  I  had 
been  accustomed  to  ascertain  the  temperature  of  dewed  grass.  But 

3* 


33  ESSAY  ON  DEW. 

observing  dew  one  night  on  the  glass  tube  of  a  thermometer,  which 
was  lying  on  a  metal  placed  upon  grass,  while  the  metal  itself  was 
free  from  moisture,  I  conceived  it  probable,  that  the  cold  then  indi- 
cated by  the  thermometer  was  not  the  real  temperature  of  the  body 
to  which  it  was  applied.  To  determine  the  point,  I  placed  on  the 
same  metal  a  second  thermometer,  covered  with  gilt  paper,  upon 
which  this  was  found  at  three  observations  to  be  6^°,  7°,  and  7° 
higher  than  the  other.  In  this  experiment,  the  bulb  of  the  naked 
thermometer  from  being  very  small  did  not  project  as  far  as  the 
outer  surface  of  the  scale,  and,  consequently,  did  not  come  in  contact 
with  the  metal.  But  even  when  the  ball  of  a  thermometer  was 
applied  directly  to  a  metal,  on  a  clear  and  calm  night,  a  temperature 
was  marked  by  it,  commonly  2  and  3,  and  sometimes  more  degrees 
less  than  that  marked  by  a  similar  thermometer,  inclosed  in  gilt 
paper,  and  similarly  placed.  I  found  it  likewise  necessary,  in  this 
inquiry,  to  correct  the  temperature  of  the  air  as  given  by  a  naked 
thermometer.  For,  on  still  and  serene  nights,  a  thermometer  in- 
closed in  a  case  of  gilt  or  silvered  paper,  and  suspended  in  the  air 
4  feet  above  the  grassplat,  was  usually  observed  to  be  l^0  or  2° 
higher  than  a  bare  thermometer  of  the  same  construction  suspend- 
ed near  to  it.  The  difference  of  two  such  thermometers,  thus 
placed,  was  once  observed  by  me  to  be  2^°,  and  once  85°.  It  may  be 
thought,  perhaps,  that  these  differences  were  caused  by  the  metalled 
case  obstructing  the  transmission  of  the  temperature  of  the  air  to 
the  inclosed  instrument.  But  that  this  was  not  the  reason  is  shown 
by  my  observing,  that  on  cloudy  nights  there  existed  no  difference 
between  the  two  thermometers ;  that  even  on  clear  nights,  a  ther- 
mometer contained  in  a  case  of  white  paper  somewhat  thicker  than 
the  metalled  was  always  nearly  of  the  same  temperature  with  a 
naked  one  which  was  suspended  close  to  it ;  and  that  when  a 
difference  did  exist  between  the  two  latter,  the  thermometer  in  the 
white  paper  case  was  commonly  lower  than  the  other. 

The  estimation  of  the  heat,  both  of  air  and  of  metals,  on  a  dewy 
night,  is  liable  to  errors  from  other  causes.  As  these,  however,  are 
trifling,  I  shall  not  mention  them,  but  proceed  to  state  the  results  of 
my  observations,  upon  the  temperature  of  metals  exposed  to  the  sky 
at  night,  though  unable  to  vouch  for  their  entire  accuracy. 

Thin  bright  metallic  plates,  the  least  having  a  surface  of  25 
square  inches,  and  some  of  them  a  surface  of  more  that  100  such 
inches,  were  several  times  observed,  while  lying  on  grass  which 
was  attracting  dew,  to  be  1  and  2,  and  once  3  degrees  warmer  than 
the  air  4  feet  above  them.  At  other  times  their  temperature 
was  the  same  with  that  of  the  air.  In  both  of  these  cases  their 
upper  surfaces  were  always  free  from  dew.  Metals  thus  situated 
were,  consequently,  often  much  warmer  than  the  grass  which  sur- 
rounded them.  I  made  no  experiments  on  this  point,  during  the 
nights  on  which  occurred  the  greatest  instances  of  cold  on  grass, 
relatively  to  the  temperature  of  the  air;  but  I  found,  notwithstand- 
ing, during  one  night,  a  metal  on  grass  to  be  10°  warmer  than  the 


THE  PHENOMENA  OF  DEW.  23 

exposed  grass  near  to  it.  On  two  other  nights  the  differences  were 
9°  and  8°.  The  superiority  of  the  heat  of  metals  on  grass  over  that 
of  the  air,  when  it  did  exist,  was  evidently  connected  with  the 
temperature  of  the  grass  which  they  covered,  and  this  again  with 
that  of  the  earth  under  the  same  portion  of  grass;  for  this  portion 
was  always  a  little  warmer  than  the  metal,  but  not  so  warm  as  the 
earth. 

On  the  other  hand,  metals,  on  which  dew  was  forming  while  they 
lay  upon  grass,  were  always  colder  than  the  air.  In  like  manner,  if 
one  metal  upon  the  grassplat  were  dewed,  while  another  similarly 
situated  remained  dry,  the  former  was  always  colder  than  the  latter. 

When  a  metal  lying  on  the  grassplat  became  dewed,  the  grass 
under  it  was  always  colder  than  that  under  another  metal,  which 
was  undewed. 

A  metal,  while  receiving  dew  in  consequence  of  being  elevated 
in  the  air,  was  always  colder  than  a  similar  metal  which  remained 
undewed  on  the  grass. 

The  greatest  instances  of  cold  observed  by  me  on  metals,  occurred 
at  times  when  other  bodies  near  to  them  had  become  considerably 
colder  than  the  atmosphere. 

The  cold,  however,  contracted  by  metals  from  exposure  to  the 
sky  in  a  clear  and  still  night,  was  always  less  than  that  of  other 
bodies  similarly  situated  ;  the  greatest  excess  of  cold  ever  observed 
by  me,  in  the  larger  metallic  plates,  from  this  cause,  over  that  of  the 
air,  being  not  more  than  3  or  4  degrees.  If  much  smaller  pieces 
were  placed  upon  grass,  the  result  was  different.  For  I  have  found 
a  small  thermometer  placed  in  this  situation,  while  inclosed  in  a 
sheath  of  gilt  paper,  to  be  only  3°  less  cold  than  the  surrounding 
grass,  during  a  night  favourable  to  the  production  of  cold  on  the 
surface  of  the  earth. 

I  collected  only  a  few  facts  respecting  the  comparative  tempera- 
tures of  different  metals,  when  they  were  exposed  together  to  the 
sky  on  dewy  nights  ;  but  such  as  I  did  collect  tend  to  prove,  that 
the  most  readily  dewed  metals  become  colder  than  the  air,  sooner 
than  those  which  receive  dew  with  greater  difficulty. 

Many  of  the  experiments  which  have  been  mentioned  in  this 
section  show,  that  when  bodies  which  had  been  equally  exposed  to 
the  night  air  were  examined  at  the  same  time,  those  which  were 
most  dewed  were  also  the  coldest.  No  such  correspondence,  how- 
ever, was  found  in  the  experiments  of  different  nights,  or  even  of 
different  parts  of  the  same  night.  Thus,  during  two  nights,  on  which 
grass  was  12°  and  14°  colder  than  the  air,  there  was  little  dew  ; 
while  on  the  night  which  afforded  the  most  copious  dew  ever  ob- 
served by  me,  the  cold  possessed  by  the  grass,  beyond  that  of  the 
air,  was  for  the  most  part  only  3°  and  4° ;  and  I  have  always  seen 
less  dew  about  sunset  than  about  sunrise,  when  the  weather  has  been 
<;alm  and  clear  at  both  times,  though  there  is  commonly,  in  this 
country  at  least,  a  greater  difference  between  the  temperature  of 
grass  and  of  air  in  the  evening  than  in  the  morning.  I  had  early 


24  ESSAY  ON  DEW. 

observed,  also,  bodies  exposed  to  the  sky,  on  a  cloudy  but  calm 
night,  to  be  sometimes  2°  or  3°  colder  than  the  air,  without  having 
any  appearance  of  dew;  and  when  two  metals  possessing  different 
relations  to  dew  were  exposed  together,  I  have  seen  the  one,  which 
was  the  fitter  to  attract  that  fluid,  colder  than  the  other,  though  both 
were  dry. 

I  shall  conclude  this  part  of  my  Essay,  with  relating  the  results 
of  some  experiments  which  were  made  for  the  purpose  of  ascertain- 
ing the  tendencies  of  various  bodies  to  become  cold  upon  exposure 
to  the  sky  at  night.  Unfortunately  the  weather  was  not  always 
favourable  to  my  views ;  but  what  occurred  appears  to  me,  notwith- 
standing, worthy  of  being  related. 

In  the  observations  hitherto  given  by  me  on  the  cold  connected 
with  dew,  the  temperature  of  grass  has  been  chiefly  considered, 
partly  because  my  first  experiments  had  been  made  upon  it,  and 
partly  from  a  wish,  which  arose  afterwards,  to  compare  my  own 
experiments  with  those  of  Mr.  Six,  which  had  been  confined  to  that 
substance.  I  found  it,  however,  very  unfit  to  furnish  the  means  of 
comparing  the  degrees  of  cold  produced  at  night  on  the  surface  of 
the  earth  at  different  times  and  places  ;  as  its  state  on  different 
nights,  on  the  same  parts  of  the  plat  I  commonly  made  use  of,  and 
in  different  parts  of  the  plat  on  the  same  nights,  was  often  very 
unequal,  in  point  of  height,  thickness  and  fineness,  all  of  which 
circumstances  influenced  the  degree  of  cold  produced  by  it.  I 
observed,  in  consequence,  a  much  greater  uniformity  in  the  results 
of  experiments  made  with  various  other  bodies,  whose  condition 
when  first  exposed  to  the  air  was  always  the  same.  Of  these,  the 
most  productive  of  cold  were  the  filamentous  and  downy,  as  wool  of 
moderate  fineness,  very  fine  raw  silk,  very  fine  unspun  cotton,  fine 
flax,  and  swandown,  all  of  which  were  not  only  more  steadily  cold, 
upon  clear  and  calm  nights,  than  grass,  but  also  gave  rise  to  a  greater 
degree  of  cold  than  was  almost  at  any  time  observed  upon  it,  even 
in  its  best  state.  Among  the  bodies  of  this  class,  wool  produced  the 
least  cold,  and  I  formerly  mentioned  that  it  attracted  less  dew  than 
silk,  cotton,  and  flax.  The  last  mentioned  substances,  and  swan- 
down,  were  found  equal,  or  nearly  so,  in  their  tendency  to  become 
cold.  Swandown,  however,  exhibited  the  greatest  cold  rather  more 
frequently  than  any  of  the  rest ;  on  which  account,  and  from  its 
being  more  easily  managed,  as  it  was  used  while  adhering  to  the 
skin  of  the  bird,  I  at  length  scarcely  ever  employed  any  other  body 
of  the  same  class.  On  the  night  during  which  grass  was  observed 
to  be  14°  colder  than  the  air,  swandown,  lying  upon  a  neighbouring 
piece  of  grass,  was  still  one  degree  lower.  The  difference  of  15°, 
between  the  temperature  at  night  of  a  body  on  the  surface  of  the 
earth,  and  that  of  the  air  a  few  feet  above  the  earth  is  the  greatest 
which  I  have  hitherto  seen. 

Fresh,  unbroken  straw,  and  shreds  of  white  paper,  though  not 
properly  to  be  ranked  among  filamentous  substances,  were  also  found 
to  be  a  little  more  productive  of  cold  than  the  wool  which  I  used. 


THE  PHENOMENA  OF  DEW.  25 

The  next  class  consisted  of  bodies  in  the  state  of  a  powder,  more 
or  less  fine.  These  were  clean  river  sand,  glass,  chalk,  charcoal, 
lampblack,  and  a  brown  calx  of  iron.  Chalk  produced  the  least,  and 
the  three  last  substances  the  greatest  cold.  They  were  all,  however, 
inferior  in  this  respect  to  bodies  of  the  first  class. 

Solid  bodies,  having  a  surface  exposed  to  the  sky  of  at  least  25 
inches  square,  formed  a  third  class  on  which  such  experiments  were 
made.  The  particular  substances  of  this  description  subjected  to 
trial,  were  glass,  brick,  cork,  oak-wood,  and  wax  ;  all  of  which  were, 
likewise,  found  inferior  to  the  filamentous  substances.  From  these 
last  experiments  it  follows,  that  when  a  glass  bulb  of  a  thermometer 
is  applied  at  night  to  a  body  exposed  to  a  clear  sky,  the  temperature 
exhibited  by  the  instrument  will  not  be  accurately  that  of  the  body 
in  question,  except  the  disposition  of  the  latter  to  become  cold  in 
such  a  situation,  be  the  same  as  that  of  glass.  An  example  of  this 
fact  has  been  given  in  this  Essay.* 

My  principal  experiments,  however,  of  this  kind  were  made  with 
snow. 

On  the  25th  of  January,  181 3,  the  ground  being  then  covered  with 
snow  about  an  inch  deep,  I  went  to  my  usual  place  of  experiment  in 
the  country  ;  but,  during  8  hours  that  I  attended  to  my  thermome- 
ters, the  whole  sky  was  constantly  overcast  with  clouds.  The 
atmosphere  was,  for  the  greater  part  of  that  time,  very  still,  and  a 
thermometer  on  the  snow  was  generally  about  2°  lower  than  another 
in  the  air.  That  this  difference  was  not  owing  to  evaporation  was 
proved  by  the  thermometer  on  the  snow  always  rising  from  a  half 
to  a  whole  degree  whenever  the  air  was  a  little  moved,  and  falling 
the  same  quantity  as  soon  as  a  great  stillness  again  took  place. 

I  had  no  opportunity  of  renewing  my  observations  upon  snow 
before  the  beginning  of  the  present  year,  1814.  The  state  of  my 
health  rendering  it  improper  that  I  should  incur  much  fatigue,  or 
be  long  exposed  to  night  air,  I  restricted  myself  to  the  making  a  few 
experiments  in  the  large  garden  in  Lincoln's-Inn  Fields.  I  went 
thither,  for  the  first  time,  on  the  evening  of  the  4th  of  January,  im- 
mediately after  a  considerable  snowfall  had  ceased,  wishing  to  begin 
my  observations  before  any  cold  should  arise  on  the  snow's  surface 
from  exposure  to  the  sky.  This  was  desirable  on  another  account ; 
for  Mr.  Kirwan,  in  direct  opposition  to  indisputable  facts  most  clearly 
stated  by  Mr.  Wilson,  has  said,  that  the  great  cold,  observed  by  that 
gentleman  on  snow,  was  occasioned  by  this  substance  having  retained 
the  temperature  of  the  high  region  from  which  it  had  fallen. t  The 
result  of  my  inquiry  was,  that  the  surface  of  the  snow  and  the  air 
4  feet  above  it  had  precisely  the  same  heat.  The  depth  of  the  snow 
was  4  inches. 

My  next  experiment  took  place  on  the  evening  of  the  6th,  the 
intervening  day  having  been  snowy.  The  sky  was  clear,  but  the 
air  had  a  considerable  motion.  The  heat  of  the  atmosphere,  at  the 

*  Page  164.  f  On  Temperatures,  p.  30. 


26  ESSAY  ON  DEW. 

height  of  4  feet,  was  at  9£h.  26°  ;  while  that  of  the  surface  of  the 
snow,  and  of  swandown  lying  upon  it,  was  22°.  The  depth  of  the 
snow  was  now  about  5  inches. 

On  the  7th,  a  little  after  sunset  the  heat  of  the  air  in  the  garden 
was  23°,  that  of  the  surface  of  snow  19,  but  that  of  swandown  lying 
upon  the  snow  only  15°.  There  was  then  a  gentle  breeze  ;  some 
parts  of  the  sky  were  covered  with  clouds,  and  the  lower  atmo- 
sphere was  a  little  obscure.  While  the  exposed  surface  of  the  snow 
was  19°,  a  part  of  its  surface,  which  had  been  covered,  about  20 
minutes  with  a  piece  of  pasteboard,  was  22°.  Grass  at  the  bottom 
of  the  snow  was  31°,  and  the  earth  an  inch  beneath  the  grass  32°. 

After  this  there  was  no  fit  time  for  observation  until  the  13th. 
The  thermometers  were  exposed  at  8h.  On  the  evening  of  that 
day,  the  sky  being  then  without  clouds  ;  but  the  stars  were  not 
bright,  and  there  was  a  perceptible  motion  in  the  air.  At  8£h. 
the  temperature  of  the  air  was  22^°,  that  of  the  surface  of  the  snow 
13°,  and  that  of  swandown  lying  on  the  snow  8°.  At  9h.  the  air 
was  23^°,  snow  17°,  and  swandown  15°.  The  sky  being  now,  in 
great  measure,  covered  with  high  thin  clouds,  my  experiments 
ceased.  At  lO^h.^the  sky  was  very  bright,  and  the  atmosphere 
very  calm  ;  but  it  was  not  then  convenient  to  me  to  renew  my  ob- 
servations. Had  I  repeated  them  at  that  time  I  should  probably 
have  found  a  difference,  between  the  temperature  of  the  swandown 
and  air,  several  degrees  more  considerable  than  the  one  of  145°, 
which  had  already  occurred  on  this  evening,  and  consequently 
greater  than  the  greatest  observed  by  Mr.  Wilson,  between  the 
temperatures  of  snow  and  of  the  atmosphere,  which  was  one  of  16°. 

The  next  favourable  evening  was  that  of  the  21st  Much  snow 
having  in  the  meanwhile  fallen,  its  depth  was  now  more  than  afoot. 
The  thermometers  were  observed  5  times  between  4h.  15m.  and 
4h.  55m.  At  4  of  those  times  the  swandown  was  13°,  and  at  one 
of  them  13^°,  colder  than  the  air,  the  heat  of  which  at  the  4  first 
observations  was  26°,  and  at  the  last  25^°.  The  temperature  of  the 
surface  of  the  snow,  during  the  whole  period  of  observation,  was  17°, 
and  consequently  4  times  it  was  4°,  and  once  5°,  less  cold  than  that 
of  the  swandown.  The  atmosphere  was  altogether  free  from  clouds, 
and  nearly  quite  calm,  but  a  good  deal  hazy. 

Before  another  proper  evening  arrived,  my  health  became  so 
infirm  that  I  was  obliged  to  relinquish  this  pursuit.  I  conclude 
therefore  my  account  of  it  with  two  remarks.  1.  If  Mr.  Wilson 
had  been  accustomed  to  examine  the  temperature  of  swandown,  or 
any  similar  substance  placed  upon  snow,  he  would  probably  have 
observed  a  cold  on  the  surface  of  the  earth  exceeding  that  of  the 
atmosphere  by  20°  or  more,  on  the  night  of  his  actually  observing 
an  excess  of  16°.  2.  Since  upon  one  evening  when  the  atmosphere 
was  neither  very  clear  nor  very  still,  a  difference  of  14^°  was  found 
by  me  between  the  temperatures  of  air  and  of  swandown,  which  is 
only  ^  a  degree  less  than  the  greatest  difference  I  have  ever  observed 
between  the  same  substances  on  the  stillest  and  clearest  nights  in 


THE  THEORY  OF  DEW.  27 

summer,  a  corroboration  is  hence  derived  of  a  conclusion,  made  by 
Mr.  Six  from  his  experiments,  that  the  greatest  differences  at  night, 
in  point  of  temperature,  between  bodies  on  the  surface  of  the  earth 
and  the  atmosphere  near  to  it,  are  those  which  take  place  in  very 
cold  weather. 


PART  II. 

OF  THE  THEORY  OF  DEW. 

DEW,  according  to  Aristotle,*  is  a  species  of  rain,  formed  in  the 
lower  atmosphere,  in  consequence  of  its  moisture  being  condensed 
by  the  cold  of  the  night  into  minute  drops.  Opinions  of  this  kind, 
respecting  the  cause  of  dew,  are  still  entertained  by  many  persons, 
among  whom  is  the  very  ingenious  Mr.  Leslie,  of  Edinburgh. t  A 
fact,  however,  first  taken  notice  of  by  Gersten,  who  published  his 
treatise  on  dew,  in  1733,  proves  them  to  be  erroneous;  for  he  found 
that  bodies  a  little  elevated  in  the  air  often  become  moist  with  dew, 
while  similar  bodies  lying  on  the  ground  remain  dry,  though, 
necessarily  from  their  position,  as  liable  to  be  wetted  by  whatever 
falls  from  the  heavens  as  the  former. 

Shortly  after  the  appearance  of  Gersten's  treatise,  Musschenbroek 
made  the  remark,  already  mentioned  in  this  Essay,  that  metals  will 
be  free  from  dew  while  other  bodies  attract  it  copiously.  This 
philosopher  contented  himself  with  publishing  his  discovery  ;  but 
his  friend  Dufay  concluded  from  it,  that  dew  is  an  electric  pheno- 
menon,since  it  leaves  untouched  the  bodies  which  conductelectricity, 
while  it  appears  upon  those  which  cannot  transmit  that  influence. 
If  dew,  however,  were  to  form  on  the  latter  only,  its  quantity 
would  never  be  sufficiently  great  to  admit  its  being  distinctly  seen  ; 
for  the  nonconductors,  as  soon  as  they  became  in  the  least  moist, 
would  be  changed  into  conductors.  Charcoal,  too,  it  is  now  known, 
though  the  best  solid  conductor  of  electricity  after  the  metals, 
attracts  dew  very  powerfully  ;  and  in  the  last  place,  contrary  to  the 
assertion  of  Dufay,  dew  frequently  forms  upon  metals  themselves. 

Other  authors  have  ascribed  the  production  of  dew  to  electricity, 
for  reasons  different  from  that  of  Dufay.  But  there  are  several 
considerations,  which  seem  to  me  to  prove  that  no  such  opinion 
can  be  just.  1.  When  dew  is  produced  in  a  clear  atmosphere,  the 
portion  of  air  by  which  it  is  deposited  must  necessarily  be  unable, 
at  that  moment  to  retain  in  a  state  of  pellucid  vapour,  all  the  mois- 
ture which  it  had  immediately  before  held  in  that  form.  But  I 
know  of  no  experiment,  which  shows  that  air,  by  becoming  posi- 

*  Meteor.  Lib.  1.  c.  x.  et  De  Mundo.  c.  iii. 

f  Relations  of  Heat  and  Moisture,  p.  37,  and  132. 


1  $ 

28  ESSAY  ON  DEW. 

tively  electrical,  which  is  said  to  be  its  condition  on  the  evenings 
during  which  dew  is  most  abundant,  is  rendered  less  able  than 
it  had  previously  been  to  contain  watery  vapour  in  a  state  of 
transparency.  2.  Bodies  in  similar  circumstances,  as  far  as  elec- 
tricity is  concerned,  acquire  very  different  quantities  of  dew. 
Wool  placed  on  the  raised  board,  for  example,  attracted  very  much 
more  dew  than  wool  attached  to  the  lower  side  of  the  same  board, 
and  even  considerably  more  than  the  same  substance  freely  suspend- 
ed in  the  air,  and  entirely  exposed  to  the  sky.  3.  Dew  forms  in 
different  parts  of  the  night,  in  quantities  no  way  proportioned  to 
the  degrees  of  electricity  found  in  the  atmosphere  at  the  same  times. 
Thus  it  is  commonly  more  copious  in  the  morning  than  in  the 
evening,  notwithstanding  that  the  air  is  observed  to  be  in  the  latter 
season  more  highly  electrical  than  in  the  former.  4.  I  have  several 
nights  held  a  glass  bottle  upon  which  dew  was  forming  close  to  the 
top  of  a  Bennett's  electrometer,  which  had  been  previously  kept  in 
a  dry  place  ;  but  I  never  saw  the  slips  of  gold  leaf  to  move  in  con- 
sequence. It  is  very  probable,  however,  that  more  refined  experi- 
ments will  show  that  electrical  appearances  attend  the  production 
of  dew.  These,  perhaps,  accompany  every  change  in  the  chemical 
form  of  bodies.  But  the  facts  which  have  been  stated,  seem  suffi- 
cient to  establish  that  any  such  appearances  which  may  be  hereafter 
remarked  during  the  formation  of  dew,  must  be  considered  as 
effects,  and  not  as  the  cause,  of  the  conversion  of  the  watery 
vapour  of  a  clear  atmosphere  into  a  fluid. 

A  remaining  argument  applies  equally  to  all  the  theories,  which 
have  hitherto  been  made  public  on  the  cause  of  dew.  This  is,  that 
none  of  them  include  the  important  fact,  that  its  production  is 
attended  with  cold  ;  since  no  explanation  of  a  natural  appearance 
can  be  well  founded,  which  has  been  built  without  the  knowledge 
of  one  of  its  principal  circumstances.  It  may  seem  strange  to 
many,  that  neither  Mr.  Wilson  nor  Mr.  Six  applied  this  fact  to  the 
improvement  of  the  theory  of  dew.  But  according  to  their  view 
of  the  subject  no  such  use  could  have  been  made  of  it  by  them,  as 
they  held  the  formation  of  that  fluid  to  be  the  cause  of  the  cold 
observed  with  it.  I  had  many  years,  as  was  formerly  mentioned, 
held  the  same  opinion  ;  but  I  began  to  see  reason,  not  long  after  my 
regular  course  of  experiments  commenced,  to  doubt  its  truth,  as  I 
found  that  bodies  would  sometimes  become  colder  than  the  air 
without  being  dewed  ;  and  that,  when  dew  was  formed,  if  different 
times  were  compared,  its  quantity,  and  the  degree  of  cold  which 
appeared  with  it,  were  very  far  from  being  always  in  the  same 
proportion  to  each  other.  The  frequent  recurrence  of  such  obser- 
vations, at  length  converted  the  doubt  of  the  justness  of  my  ancient 
opinion  into  a  conviction  of  its  error,  and  at  the  same  time  occa- 
sioned me  to  conclude,  that  dew  is  the  production  of  a  preceding 
cold  in  the  substances  upon  which  it  appears.  Wishing,  however, 
to  obtain  proofs,  more  striking  in  degree,  of  the  validity  of  these 
inferences,  than  such  as  had  been  afforded  to  me  by  casual  observa- 


THE  THEORY  OF  DEW.  „        29 

tion  while  attending  to  other  parts  of  my  subject,  I  instituted  the 
experiments  which  will  be  next  related. 

I  had  frequently  remarked,  early  in  the  evening,  a  considerable 
degree  of  cold  on  substances  exposed  in  calm  weather  to  a  clear  sky, 
and  I  had  also  sometimes  seen,  early  in  the  evening,  the  raised  board 
altogether  dry,  while  the  grass  was  much  moistened.  I  therefore 
determined  to  make  the  experiments  in  view  on  the  raised  board,  and 
to  commence  them  as  soon  as  the  sun  should  cease  to  shine  upon  it. 
The  first  day  I  went  to  the  country  for  this  purpose,  the  19th  of 
August,  1813,  almost  every  circumstance  was  favourable  to  its 
completion.  There  had  been  no  rain  for  three  weeks ;  the  wind 
was  northerly ;  and  the  barometer  was  rising ;  all  which  indicated, 
that  the  atmosphere  contained  little  moisture.  The  air  too  was 
extremely  still.  The  only  appearance  in  the  least  unfavourable  was, 
that  the  sky  was  not  entirely  free  from  clouds ;  but  these  were  few, 
of  small  extent,  thin,  and  high. 

At  6h.  25m.  immediately  after  the  sun  had  ceased  to  shine  upon 
the  spot,  where  my  experiments  were  to  be  carried  on,  though  the 
time  of  its  setting  was  still  47  minutes  distant,  I  placed  upon  the 
raised  board  10  grains  of  woo),  and  a  small  bag,  made  of  the  skin 
of  a  swan's  breast  with  the  down  adhering,  and  stuffed  with  wool,  the 
whole  weighing  nearly  5  drachms.  On  each  of  these  substances  the 
naked  bulb  of  a  small  and  delicate  thermometer  was  laid.  A  simi- 
lar thermometer,  with  its  bulb  also  naked,  was  suspended  in  the  air 
over  the  grassplat,  at  the  same  height  with  the  board.  Two  ther- 
mometers were  placed  in  other  situations,  as  will  be  seen  in  the 
annexed  table.  After  an  exposure  of  20  minutes,  the  wool  was  7° 
colder  than  the  air,  but  the  swandown  bag  only  6°,  no  doubt  in 
consequence  of  its  comparatively  great  quantity  of  matter.  Neither, 
however,  had  gained  the  least  weight,  according  to  the  scales 
employed  by  me,  which  were  sensibly  moved  by  the  16th  of  a 
grain.  These  observations  were  repeated  several  times  during  the 
following  hour,  as  will  be  seen  by  the  table,  at  none  of  which, 
except  the  last,  was  either  the  wool  or  swandown  found  in  the  least 
heavier,  than  when  first  placed  on  the  board.  At  this  last  observa- 
tion, the  wool,  though  9|°  colder  than  the  air,  was  still  without  any 
increase  in  weight  ;  but  the  swandown  which  was  1°  colder  than 
the  wool,  had  gained  i  a  grain.  My  experiments  now  properly 
ceased;  but  having  suffered  the  thermometers,  which  had  been 
placed  on  the  wool  and  swandown,  and  in  the  air,  to  remain  in  those 
situations,  .1  examined  them  again  at  Sh.  45m.,  that  is,  2h.  20m. 
after  they  had  been  first  exposed.  The  wool,  which  was  still  9£° 
colder  than  the  air,  had  gained  somewhat  less  than  I  a  grain  ;  and 
the  swandown,  which  was  now  11^  colder  than  the  air,  had  gained 
2  grains,  including  the  5  grain  already  mentioned.  When  these 
last  observations  were  made,  the  sky  was  entirely  cloudless,  and 
the  atmosphere  very  calm. 

OCT.  1838.-— S  4 


30 


ESSAY  ON  DEW. 


TABULAR    VIEW    OP    OBSERVATIONS    ON   THE    EVENING   OF 
AUGUST    19,    1813. 


6h.  45m. 

7h. 

7h.  20m. 

7h.  40m. 

8h.  45m. 

Heat  of  air  4  feet  above  the  grass, 

60io 

60£° 

59° 

58° 

54° 

wool  on  the  raised  board, 

53£ 

54£ 

51$ 

48i 

44$ 

swandown  on  the  same, 

54£ 

53 

51 

47$ 

42^ 

surface  of  the  raised  board, 

58 

57 

55£ 

grassplat,*     - 

53 

51 

49£ 

49 

42 

Similar  experiments  made  at  the  same  place,  on  the  evenings  of 
the  25th  of  August  and  17th  of  September,  in  the  same  year,  had 
results,  which  were  also  similar  but  less  in  degree  ;  the  greatest 
difference  between  the  temperature  of  wool  or  swandown,  while 
they  were  without  any  increase  of  weight,  and  the  temperature  of 
the  air  having  been,  on  the  first  of  those  evenings,  only  4°,  and  on 
the  second  only  5°.  The  reasons  were,  in  great  measure,  if  not 
wholly,  that  a  considerable  part  of  the  sky  was  covered  with  clouds, 
and  that  the  air  was  commonly  in  that  state  of  motion  which  is 
denominated  a  gentle  breeze. 

On  the  evening  of  my  first  experiments,  I  had  omitted  to  measure 
the  heat  of  the  raised  board,  before  the  thermometers  were  placed 
upon  it.  This  was  attended  to  on  the  two  latter  evenings,  on  the 
first  of  which  its  upper  surface  was  found,  at  the  commencement  of 
the  experiments,  4°  warmer  than  the  air ;  on  the  second,  both  it 
and  the  air  were  of  the  same  temperature.  Again  ;  on  the  first  of 
the  latter  evenings,  10  grains  of  wool,  to  which  three  grains  of 
water  had  been  added,  having  been  laid  on  the  raised  board,  near 
the  thermometers  ;  at  the  end  of  45  minutes  the  parcel  was  found 
to  have  lost  2%  grains  of  moisture  by  evaporation  during  the  time 
that  dry  wool  had  become  several  degrees  colder  than  the  air. 

A  fourth  experiment  of  this  kind  was  made  by  me  on  the  7th  of 
January,  1814,  in  the  garden  of  Lincoln's-Inn-Fields,  by  placing 
10  grains  of  wool  on  a  sheet  of  pasteboard,  which  lay  upon  the 
snow.  At  the  end  of  35  minutes  the  wool  was  5°  colder  than  the 
air,  without  possessing  any  additional  weight. 

I  took  advantage  of  being  in  the  country,  at  the  distance  of  a  few 
miles  from  London ,  on  the  2 1  st  of  the  present  month,  the  last  day  but 
one  of  an  unusually  long' tract  of  dry  weather,  to  expose  to  the  sky,  28 
minutes  beforetsunset,  weighed  parcels  of  wool  and  swandown,  upon 
a  smooth,  unpainted,  and  perfectly  dry  fir-table,  5  feet  long,  3  broad, 
and  nearly  3  in  height,  which  had  been  placed  an  hour  before  in  the 
sunshine,  in  a  large  level  grass-field.  At  this  time,  and  throughout 
my  experiments,  the  air  was  very  still,  and  the  sky  very  serene. 

*  In  these  experiments,  contrary  to  what  usually  happens,  the  grass  was 
almost  constantly  colder  than  the  filamentous  substances,  although  they  were 
placed  upon  the  raised  board. 


,       THE  THEORY  OF  DEW.  31 

The  atmosphere,  too,  in  all  probability,  contained  but  little  moisture 
in  consequence  of  the  long  absence  of  rain  ;  and  the  surface  of  the 
ground  apparently  contained  none.  The  wool,  12  minutes  after 
sunset,  was  found  to  be  14°  colder  than  the  air,  the  temperature  of  the 
latter  being  measured  by  a  naked  thermometer  suspended  4  feet  above 
the  ground,  and  to  have  acquired  no  weight.  The  swandown,  the 
quantity  of  which  was  much  greater  than  that  of  the  wool,  was  at 
the  same  time  13°  colder  than  the  air,  and  was  also  without  any 
additional  weight.  In  20  minutes  more,  the  swandown  was  14^° 
colder  than  the  neighbouring  air,  and  was  still  without  any  increase 
of  its  weight.  My  experiments  now  ceased  from  a  failure  of  daylight. 

In  my  former  experiments  of  this  kind,  the  greatest  cold  observed 
by  me  from  radiation,  without  the  appearance  of  dew,  was  only  9£°. 

While  making  the  experiments  on  wool  and  swandown,  I  attended 
frequently  to  the  temperature  of  the  grass,  and  found  it  at  one  time 
15°  colder  than  that  of  the  air  4  feet  above  the  ground.  This  differ- 
ence is  1°  greater  than  any  I  had  ever  before  seen  between  the 
temperatures  of  the  same  substances,  and  is  equal  to  the  greatest 
which  I  had  ever  known  to  occur,  between  those  of  the  atmosphere 
and  of  swandown  lying  upon  grass.  I  had  this  evening  placed  no 
swandown  upon  grass. 

Having  thus  shown  the  justness  of  my  former  conclusion,  that 
the  cold  observed  with  dew  is  the  previous  occurrence,  and  conse- 
quently, that  the  formation  of  this  fluid  has  precisely  the  same  im- 
mediate cause,  as  the  presence  of  moisture  upon  the  outside  of  a 
glass  or  metallic  vessel,  when  a  liquid  considerably  colder  than  the 
air  has  been  poured  into  it  shortly  before  ;  I  shall  next  apply  this 
fact  to  the  explanation  of  several  atmospherical  appearances. 

I.  The  variety  in  the  quantities  of  dew,  which  were  found  by 
me  upon  bodies  of  the  same  kind,  exposed  to  the  air  during  the 
same  time  of  the  night,  but  in  different  situations,  is  now  seen  to 
have  been  occasioned  by  the  diversity  of  temperature  which  existed 
among  them. 

II.  Agreeably  to  the  opinion  of  Mr.  Wilson  and  Mr.  Six,  the 
cold  connected  with  dew  ought  always  to  be  proportional  to  the 
quantity  of  that  fluid  ;  but  this  is  contradicted  by  experience.     On 
the  other  hand  if  it  be  granted,  that  dew  is  water  precipitated  from 
the  atmosphere,  by  the  cold  of  the  body  on  which  it  appears,  the 
same  degree  of  cold,  in  the  precipitating  body,  may  be  attended  with 
much,  with  little,  or  with  no  dew,  according  to  the  existing  state 
of  the  air  in  regard  to  moisture  ;  all  of  which  circumstances  are 
found  actually  to  take  place. 

III.  The  formation  of  dew,  indeed,  not  only  does  not  produce 
cold,  but  like  every  other  precipitation  of  water  from  the  atmosphere 
produces  heat.     I  infer  this,  partly  because  very  little  dew  appeared 
upon  the  two  nights  of  the  greatest  cold  I  have  ever  observed  on 
the  surface  of  the  earth,  relatively  to  the  temperature  of  the  air, 
both  of  them  having  occurred  after  a  long  tract  of  dry  weather  ;  and 
partly  from  the  most  dewy  night,  which  I  have  ever  seen,  having 


32  ESSAY  ON  DEW. 

been  attended  during  the  greater  part  of  it  with  no  considerable 
degree  of  cold.  On  this  night  the  difference  between  the  tempera- 
tures of  grass  and  of  air  was  at  first  7$,  the  dew  being  then  not 
very  abundant.  But  after  the  dew  had  become  very  abundant,  the 
difference  of  those  temperatures  never  exceeded  4°,  and  was  fre- 
quently only  3°. 

With  the  view  of  obtaining,  though  indirectly,  some  knowledge 
of  the  quantity  of  cold,  which  had  been  prevented,  by  the  formation 
of  dew,  from  appearing  on  the  surface  of  the  earth  in  the  night  just 
spoken  of,  I  made  the  following  experiment.  To  10  grains  of  wool 
having  the  same  form  and  extension  as  the  parcels  employed  for 
the  collection  of  that  fluid,  were  added  21  grains  of  water,  this 
being  the  quantity  of  moisture,  which  had  been  attracted  by  10 
grains  of  wool,  lying  on  the  grassplat,  in  the  space  of  8  hours  on 
that  night.  The  wet  wool  having  been  then  placed  in  a  china 
saucer,  laid  on  a  feather-bed  in  the  room,  the  door  and  windows  of 
which  were  shut,  its  heat  during  the  following  8  hours  was,  at  fre- 
quent examinations,  uniformly  found  to  be  about  4°  less  than  that 
of  a  dry  china  saucer  on  the  same  bed  ;  the  temperature  of  the  air 
in  the  room  not  having  altered  more  than  §  a  degree  in  the  course 
of  the  experiment.  At  the  end  of  the  8  hours,  the  wool  still  re- 
tained 2^  grains  of  moisture.  If  this  quantity  had  also  evaporated 
the  cold  uniformly  produced  during  8  hours  would,  in  all  probability, 
have  been  about  4^°  From  this  experiment,  therefore,  1  think  it 
may  be  inferred,  that  the  mean  quantity  of  cold,  which  was  pre- 
vented, by  the  formation  of  dew,  from  appearing  on  the  ground, 
during  the  night  which  has  been  mentioned,  was  also  about  4^°. 
But  as  the  production  of  dew,  during  some  parts  of  the  night,  was 
at  at  a  greater  rate  than  that  of  21  grains  for  eight  hours,  1  or  2 
degrees  may  be  added  for  those  times,  which  will  raise  the  effect  of 
the  dew  in  diminishing  the  appearance  of  cold  during  them  to  about 
6°,  on  the  supposition,  which  cannot  be  far  from  the  truth,  that  dew 
had  been  attracted  as  copiously  by  the  grass,  as  by  wool  which  lay 
upon  it. 

The  less  difference  commonly  observed  between  the  temperatures 
of  grass  and  of  air  in  the  morning  than  what  occurs  in  the  evening, 
is  likewise  to  be,  in  part,  attributed  to  a  greater  quantity  of  dew 
appearing  in  the  former  than  in  the  latter  season. 

A  more  remarkable  fact,  deriving  an  explanation  from  the  same 
source,  is  the  greater  difference  which  takes  place  in  very  cold 
weather,  if  it  be  calm  and  clear,  between  the  temperatures  of  the  air 
and  of  bodies  on  the  earth,  at  night,  than  in  equally  clear  and  calm 
weather  in  summer  ;  since,  in  very  cold  weather,  any  diminution 
of  the  temperature  of  a  portion  of  air,  in  contact  with  a  cold  body, 
will  be  attended,  in  consequence  of  the  well  known  relations  of  the 
atmosphere  to  moisture,  with  a  much  less  formation  of  water  than 
an  equal  diminution  would  be  in  summer,  supposing  the  air,  before 
it  touches  the  cold  body,  to  be  at  both  times  equally  near  to  its 
point  of  repletion  with  moisture. 


THE  THEORY  OF  DEW.  33 

IV.  In  very  calm  nights,  a  portion  of  air  which  comes  in  contact 
with  cold  grass,  will  not,  when  the  surface  is  level,  immediately 
quit  it,  more  especially  as  this  air  has  become  specifically  heavier 
than  the  higher,  from  a  diminution  of  its  heat,  but  will  proceed  hori- 
zontally, and  be  applied  successively  to  different  parts  of  the  same 
surface.     The  air,  therefore,  which  makes  this  progress,  must  at 
length  have  no  moisture  to  be  precipitated,  unless  the  cold  of  the 
grass  which  it  touches  should  increase.     Hence  in  great  measure  is 
to  be  explained,  why  on  such  nights  as  have  been  just  mentioned, 
more  dew  was  acquired  by  substances  placed  on  the  raised  board, 
than  by  others  of  the  same  kind  on  the  grass,  though  it  began  to 
form  much  sooner  in  the  latter  than  in  the  former  situation,  those 
on  the  raised  board  having  received  air,  which  had  previously  de- 
posited less  of  its  moisture. 

A  reason  is  now  also  afforded,  why  a  slight  agitation  of  the  atmo- 
sphere, when  very  pregnant  with'jnoisture,  should  increase  the  quan- 
tity of  dew ;  since  fresh  parcels  of  air  will  hence  be  more  frequently 
brought  into  contact  with  the  cold  surface  of  the  earth  than  if  the 
atmosphere  were  entirely  calm. 

V.  Dew,  in  agreement  with  the  immediate  cause  which  has  been 
assigned  by  me  for  its  production,  can  never  be  formed,  in  temperate 
climates,  upon  the  naked  parts  of  a  living  and  healthy  human  body 
during  the  night;  since  their  heat  is  never  less  in  this  season,  in 
such  climates,  than  that  of  the  atmosphere.     I  have,  in  fact,  never 
perceived  dew  on  any  naked  part  of  my  own  body  at  night,  though 
my  attention  was  much  occupied,  for  three  years,  with  every  thing 
relative  to  this  fluid,  and  though  I  had  been  during  that  period, 
much  exposed  to  the  night  air.     On  the  other  hand,  in  very  hot 
countries  the  uncovered  parts  of  a  human  body  may  sometimes, 
from  being  considerably  colder  than  the  air,  condense  the  watery 
vapour  of  the  atmosphere,  and  hence  be  covered  with  a  real  dew, 
even  in  the  day-time. 

VI.  Hygrometers  formed  of  animal  or  vegetable  substances,  when 
exposed  to  a  clear  sky  at  night,  will  become  colder  than  the  atmo- 
sphere ;  and  hence  by  attracting  dew,  or  according  to  an  observation 
of  Saussure,*  by  merely  cooling  the  air  contiguous  to  them,  mark  a 
degree  of  moisture  beyond  what  the  atmosphere  actually  contains. 
This  serves  to  explain  an  observation  made  by  Mr.  De  Luc,t  that 
in  serene  and  calm  weather,  the  humidity  of  the  air,  as  determined 
by  an  hygrometer,  increases  about,  and  after  sunset  with  a  greater 
rapidity,  than  can  be  attributed  to  a  diminution  of  the  general  heat 
of  the  atmosphere. 

-These  examples  are  sufficient  to  show  the  value  of  the  fact,  that 
bodies  become  colder  than  the  neighbouring  air,  before  they  are 
dewed,  in  explaining  many  atmospherical  appearances.  To  this 
point,  the  investigation  of  the  cause  of  dew  might  have  been  carried 
at  any  time  since  the  invention  of  thermometers ;  but  its  complete 

*  Hygronometrie,  p.  25.       f  Introduction  a  la  Physique  Terrestre,  II.  491. 

4* 


34  ESSAY  ON  DEW. 

theory  could  not  possibly,  in  my  opinion,  have  been  attained  before 
the  discoveries  on  heat  were  made,  which  are  contained  in  the 
works  of  Mr.  Leslie  and  Count  Rumford. 

The  experience  of  most  persons,  respecting  the  communication 
of  heat  among  bodies  in  the  open  air,  is  confined  to  what  happens 
during  the  day ;  at  which  time,  those  that  are  situated  near  to  one 
another  are  always  found  to  possess  the  same  temperature,  unless 
some  very  evident  reason  for  the  contrary  should  exist.  To  many, 
therefore,  it  may  appear  incredible  that  a  perfectly  dry  body,  placed 
in  contact  on  all  sides,  with  other  bodies  of  the  same  temperature 
with  itself,  shall  afterwards  without  undergoing  any  chemical  change, 
become  much  colder  than  they  are,  and  shall  remain  so  for  many 
hours;  yet  these  circumstances  are  found  to  occur  in  substances  at- 
tractive of  dew,  when  laid  on  the  surface  of  the  earth,  in  a  still  and 
serene  night,  and  are  in  perfect  agreement  with  the  doctrine  of  heat, 
now  universally  admitted  to  be  just. 

To  render  this  more  easy  of  apprehension,  let  a  small  body  which 
radiates  heat  freely,  and  possesses  a  temperature  in  common  with 
the  atmosphere,  higher  than  32°,  be  placed  while  the  air  is  clear  and 
still,  on  a  slow  conductor  of  heat  lying  on  the  surface  of  a  large  open 
plain,  and  let  a  firmament  of  ice  be  supposed  to  exist  at  any  height 
in  the  atmosphere;  the  consequence  must  be,  that  the  small  body 
will  from  its  situation  quickly  become  colder  than  the  neighbouring 
air.  For,  while  it  radiates  its  own  heat  upwards,  it  cannot  receive 
a  sufficient  quantity  from  the  ice  to  compensate  this  loss;  little  also 
can  be  conveyed  to  it  from  the  earth,  as  a  bad  conductor  is  interposed 
between  them ;  and  there  is  no  solid,  or  fluid  except  the  air,  to  com- 
municate it  laterally  either  by  radiation  or  conduction.  This  small 
body,  therefore,  unless  it  shall  receive  from  the  air  nearly  as  much 
heat  as  it  has  emitted,  which,  considering  the  little  that  can  be  com- 
municated from  one  part  of  the  atmosphere  to  another  in  its  present 
calm  state,  must  be  regarded  as  impossible,  will  become  colder  than 
the  air,  and  condense  the  watery  vapour  of  the  contiguous  parts  of 
it,  if  they  should  contain  a  sufficient  quantity  to  admit  of  this  effect. 
But  events  similar  to  these  occur  when  dew  appears  in  an  open  and 
level  grass  field  during  a  still  and  serene  night.  The  upper  parts  of 
the  grass  radiate  their  heat  into  regions  of  empty  space,  which  con- 
sequently send  back  no  heat  in  return ;  its  lower  parts  from  the 
smallness  of  their  conducting  power,  transmit  little  of  the  earth's 
heat  to  the  upper  parts,  which  at  the  same  time  receiving  only  a 
small  quantity  from  the  atmosphere,  and  none  from  any  other  lateral 
body,  must  remain  colder  than  the  air,  and  condense  into  dew  its 
watery  vapour,  if  this  be  sufficiently  abundant,  in  respect  to  the  de- 
creased temperature  of  the  grass.* 

*  I  have  adopted  in  this  explanation  the  hypothesis  of  Mr.  Prevost  of  Geneva, 
on  the  constant  radiation  of  heat  by  bodies  in  contact  with  the  atmosphere,  even 
at  the  time  .that  they  are  exposed  to  the  influence  of  bodies  warmer  than  them- 
selves; as  it  appears  to  agree  perfectly  with  all  the  phenomena  of  the  communi- 
cation of  heat,  which  do  not  depend  upon  conduction.  I  shall  hereafter  make 
frequent  use  of  this  hypothesis. 


THE  THEORY  OF  DEW.  35 

/  '  *,  "•' 

This  subject  may  be  further  illustrated  by  a  reference  to  what 
happens  in  the  experiment,  which  has  been  used  to  prove  the  reflec- 
tion of  cold. 

In  the  simplest  form  of  this  experiment,  a  small  body,  the  bulb  of 
a  thermometer,  possessing  the  temperature  of  the  atmosphere,  is 
placed  before  a  larger  cold  body,  rendered  equal  in  effect  to  one  still 
larger,  by  means  of  a  concave  metallic  mirror.  In  this  situation, 
the  small  body  radiates  heat  to  the  larger,  without  receiving  an 
equivalent  from  it,  and,  in  consequence,  becomes  colder  than  the  air 
through  which  its  heat  is  sent,  notwithstanding  that  it  is  continually 
gaining  some  heat,  both  from  the  air  which  surrounds  it,  and  from 
the  walls  and  contents  of  the  apartment  in  which  the  experiment 
is  made.  Dew,  therefore,  would  as  readily  form  upon  the  thermo- 
meter in  this  experiment,  as  it  would  upon  one  suspended  in  the 
open  air  at  night,  under  a  clear  sky,  provided  that  the  two  instru- 
ments were  equally  colder  than  the  atmosphere,  and  that  this  was 
in  both  cases  equally  near  to  being  replete  with  moisture.* 

Regarding  now  as  established,  that  bodies  situated  on  or  near  to 
the  surface  of  the  earth  become,  under  certain  circumstances,  colder 
than  the  neighbouring  air,  by  radiating  more  heat  to  the  heavens 
than  they  receive  in  every  way,t  I  shall  in  the  first  place  offer  a 
few  remarks  on  the  extent  and  use  of  this  occurrence,  and  shall 
afterwards  apply  the  knowledge  of  it  to  the  explanation  of  several 
more  of  the  appearances  described  in  the  former  part  of  this  Essay, 
and  of  some  others  which  have  not  hitherto  been  mentioned  by  me. 

Radiation  of  heat  by  the  earth  to  the  heavens  must  exist  at  all 
times  ;  but,  if  the  sun  be  at  some  height  above  the  horizon,  the 
degree  of  which  is  hitherto  undetermined,  and  probably  varies 
according  to  season,  and  several  other  circumstances,  the  heat  emit- 
ted by  it  to  the  earth  will  overbalance,  even  in  places  shaded  from 

*  The  invention  of  this  experiment  having  been  ascribed  a  few  years  ago  to  Mr. 
Pictet  of  Geneva,  various  English  writers  have  shown,  that  it  occurs  in  several 
much  older  foreign  authors.  But  I  have  not  seen  any  mention  made  of  its  having 
been  also  long  since  known  in  this  country.  That  it  was  so  appears  from  the 
following  extract  of  a  letter,  written  by  Mr.  Oldenburgh  to  Mr.  Boyle  in  1665. 
"  I  met  the  other  day  in  the  Astrological  Discourse  of  Sir  Christopher  Heydon, 
with  an  experiment,  which  he  affirms  to  have  tried  himself,  importing,  that  cold 
accompanies  reflected  light,  by  employing  burning  spherical  concaves,  or  para- 
bolical sections,  which,  he  saith,  will  as  sensibly  reflect  the  actual  cold  of  snow 
or  ice,  as  they  will  the  heat  of  the  sun." — Boyle's  Works,  folio,  vol.  v.  p.  345. 

|  Count  Rumford  offered  the  following  conjecture,  in  a  paper  printed  in  the 
Philosophical  Transactions  for  1804.  "  The  excessive  cold  which  is  known  to 
reign,  in  all  seasons  on  the  tops  of  very  high  mountains,  and  in  the  higher  regions 
of  the  atmosphere,  and  the  frosts  at  night,  which  so  frequently  take  place  on  the 
surface  of  the  plains  below,  in  very  clear  and  still  weather,  in  spring  and  autumn, 
seem  to  indicate,  that  frigorific  rays  arrive  continually  at  the  surface  of  the  earth, 
from  every  part  of  the  heavens."  But  he  gave  no  experiments  to  prove,  that 
such  a  communication  actually  exists  between  the  heavens  and  the  earth  at  night. 
Neither  does  it  appear  from  any  of  his  writings  which  I  have  seen,  that  he  ever 
supposed,  that  the  surface  of  the  earth  is  more  cooled  by  these  frigorific  rays, 
than  the  air  through  which  they  pass,  or  that  some  solid  bodies  are  more  cooled 
by  them  than  others. 


36  ESSAY  ON  DEW. 

,. 

its  direct  beams,  that  which  the  earth  radiates  upwards.  I  sus- 
pended at  midday,  on  the  24th  of  July,  1813,  in  the  open  air  over 
a  grassplat,  while  the  sky  was  wholly  covered  with  very  dense 
clouds,  and  the  weather  calm,  two  delicate  thermometers,  one  of 
which  was  naked,  but  the  other  cased  in  gold  paper.  At  two  ob- 
servations, having  an  interval  of  10  minutes  between  them,  the  ther- 
mometer in  the  gilt  case  was  2°  lower  than  that  which  was  naked. 
A  white  paper  case  was  then  drawn  over  the  gilt  one,  upon  which, 
after  5  minutes,  the  covered  instrument  was  observed  to  be  at  the 
same  height  with  the  naked.  The  outer  white  case  having,  in  the 
next  place,  been  taken  from  the  covered  thermometer,  but  that  which 
was  gilt  suffered  to  remain,  the  two  instruments  were  in  a  few  mi- 
nutes found  again  to  differ  2°.  A  thermometer  on  the  grassplat  was, 
during  these  experiments,  higher  than  the  naked  instrument  in  the 
air  by  2°,  and  than  that  in  the  gilt  case  by  4°.  It  is  evident,  therefore, 
that  heat  radiated  by  the  sun  must,  on  this  day,  have  been  transmit- 
ted in  considerably  quantity  through  the  thickest  clouds  ;  since  not 
only  was  the  earth's  surface  warmer  than  the  air,  but  a  small  body, 
covered  with  a  substance  not  readily  admitting  the  entrance  of 
radiant  heat  was  colder  than  a  similar  body  which  was  uncovered. 
In  like  manner,  I  observed  at  noon,  on  the  2d  of  January,  1814, 
during  the  prevalence  of  a  dense  fog,  a  thermometer  placed  upon 
swandown,  which  was  lying  upon  grass  thickly  incrusted  with 
hoarfrost,  to  be  2°  warmer  than  the  air,  and  1°  warmer  than  the 
grass.* 

In  a  calm  and  serene  night,  however,  when  consequently  little 
impediment  exists  to  the  escape,  by  radiation,  of  the  earth's  heat  to 
the  heavens,  and  when  no  heat  can  be  radiated  by  the  sun  to  the 
place  of  observation,  an  immense  degree  of  cold  would  occur  on  the 
ground,  if  the  following  circumstances  did  not  combine  to  lessen  it. 

1.  The  incapacity  of  all  bodies  to  prevent,  entirely,  the  passing  of 
heat,  by  conduction,  from  the  earth  to  substances  placed  upon  them. 

2.  The  heat  radiated  to  these  substances  by  lateral  objects.     3.  The 
heat  communicated  to  the  same  substances  by  the  air.     4.  The  heat 
which  is  evolved,  during  the  condensation  of  the  watery  vapour  of 
the  atmosphere  into  dew. 

The  extent  of  the  effect  of  all  these  checks  upon  the  production 
of  cold,  by  the  nightly  radiation  of  heat  from  bodies  on  the  surface 
of  the  earth,  cannot,  in  the  present  state  of  our  knowledge,  be  pro- 
perly estimated  ;  but  facts  show  that,  notwithstanding  theiroperation, 
the  cold  originating  in  this  source  must  be  often  very  considerable. 

1.  Mr.  Wilson  once  observed  a  difference  of  16°,  from  this  cause, 
between  the  temperatures  of  snow  and  of  air.  In  taking  the  latter 
temperature,  however,  he  employed  a  naked  thermometer,  on  which 
account,  in  consequence  of  what  has  already  been  mentioned  by  me, 

*  Another  fact  of  the  same  kind,  which  occurred  at  the  same  time,  is  that, 
although  the  temperature  of  the  air  was  30°,  the  hoarfrost  on  trees  rapidly  de- 
creased, the  solid  matter  of  the  trees  intercepting  radiant  heat,  which  had  pene- 
trated through  the  fog  from  the  sun,  and  converting  it  into  heat  of  temperature. 


THE  THEORY  OF  DEW.  37 

about  2°  are  to  be  added  to  the  16°  noted  by  him,  in  order  to  obtain 
the  real  difference  between  the  heat  of  the  snow  and  the  air  at  that 
time.* 

2.  If  Mr.  Wilson,  as  was  formerly  said,  had  laid  a  thermometer 
on  any  downy  substance  in  contact  with  the  snow,  he  would,  in  all 
probability,  have  found  a  cold  indicated  by  it  at  least  20°  greater 
than  that  of  the  air,  as  marked   by  a  naked  instrument,  and  conse- 
quently at  least  22°  greater  than  the  real  cold  of  the  surrounding 
atmosphere. 

3.  Mr.  Wilson's  place  of  observation  was  not  very  favourable  to 
the  occurrence  of  a  great  cold,  from   radiation  of  heat  at  night,  it 
being  near  to  a  large  smoky  city,  in  the  immediate  vicinity  also,  as 
appears  to  me  from  what  he  says  of  it,  of  one  or  more  considerable 
buildings,  and  in  a  climate  abounding  in  moisture. 

4.  None  of  Mr.  Wilson's  experiments,  in   which   a  very  great 
degree  of  cold  occurred,  were  made  within  an  hour  or  two  after 
sunset,  during  which  time,  according  to  my  observation,  the  most 
considerable  differences  between  the  temperatures  of  the  air,  and  of 
bodies  on  the  surface  of  the  earth,  commonly  happen. 

If,  then,  such  experiments  should  be  made  in  an  atmosphere  still 
colder  than  that  in  which  Mr.  Wilson  made  his,  on  a  large  plain 
remote  from  any  city,  and  free  from  objects  of  every  kind  that  are 
elevated  above  the  ground,  and  in  a  country  remarkable  for  the 
dryness  of  its  air,  all  which  circumstances  may  be  found  in  Russia 
during  the  winter  ;  a  difference  of  at  least  30°  would  probably  ap- 
pear, on  some  still  and  serene  night,  between  a  small  thermometer 
placed  with  its  bulb  naked,t  on  the  middle,  or  leeward  side  of  a 
stratum  of  a  downy  substance,  occupying  a  space  upon  a  grass  field, 
or  bed  of  snow,  one  or  two  square  yards  in  extent,  and  a  similar 
thermometer  inclosed  in  a  case  of  gilt  paper,  and  suspended  in  the 
air  a  few  feet  above  the  other.  Two  thermometers,  thus  placed, 
would,  I  think,  be  sometimes  found  even  in  this  country  to  differ 
not  much  less  than  30°.  I  have  myself  never  made  any  such  ex- 
periments with  a  downy  substance,  which  had  a  surface  of  more 
than  a  few  square  inches,  or  in  a  very  cold  night,  when  the  atmo- 
sphere was  clear  and  calm,  and  the  scene  of  observation  remote  from 
large  masses  of  building. 

But  even  a  cold  of  30°  appears  not  to  be  the  greatest  that  can  be 
thought  to  occur  from  the  radiation  of  heat  to  the  heavens,  at  night, 
by  substances  on  the  surface  of  the  earth.  For  experiments  by  Mr. 
Pictet,  J  Mr.  Six,§  and  I  may  add  by  myself,  establish  that,  in  ex- 

*  As  bright  metals,  when  suspended  in  the  air,  and  exposed  to  a  clear  sky  on 
a  calm  night,  become  colder  than  the  surrounding  atmosphere,  a  thermometer 
covered  with  metalled  paper,  and  placed  in  the  circumstances  which  have  been 
just  mentioned,  will  mark  a  temperature  less  than  that  of  the  air  near  to  it.  But, 
as  the  difference  must  be  small,  and  as  I  know  of  no  way  to  estimate  it  accurately, 
I  have  hitherto  always  neglected  to  consider  it. 

f  The  effect  would,  perhaps,  be  a  little  increased,  by  covering  the  bulb  with  a 
very  thin  layer  of  lamp-black. 

|  Essai  sur  le  Feu,  c.  x.  §  Phil.  Trans.  1784,  and  1788. 


38  ESSAY  ON  DEW. 

ception  to  the  common  rule,  the  heat  of  the  atmosphere  in  clear  and 
calm  nights  increases  with  the  distance  from  the  earth.  Agreeably 
to  Mr.  Six's  experiments,  the  atmosphere  at  the  height  of  220  feet 
is  often,  upon  such  nights,  10°  warmer  than  what  it  is  7  feet  above 
the  ground.  If,  therefore,  I  am  able  to  show,  as  I  expect  I  shall  be 
in  the  course  of  a  few  pages,  that  the  air  at  the  smaller  height  be- 
comes colder  than  that  of  the  greater,  from  its  vicinity  to  the  surface 
of  the  earth,  previously  rendered  cold  by  radiating  its  heat  to  the 
heavens,  it  will  follow,  that  these  10°  must  be  added  to  the  quantity 
of  cold  already  mentioned  ;  and,  consequently,  that  a  body  on  the 
ground  may  become  at  night,  at  least  40°  colder  than  the  air  two  or 
three  hundred  feet  above  it,  by  the  radiation  of  its  heat  to  a  clear 
sky, 

I  shall  add,  with  the  greatest  diffidence,  a  few  words  upon  a  final 
cause  of  the  radiation  of  heat  from  the  earth,  at  night,  and  upon 
some  of  the  circumstances  which  modify  its  action,  though  fully 
conscious  of  the  danger  of  error  which  is  always  incurred  in  the 
attempt  to  appreciate  the  works  of  our  Creator. 

The  heat  which  is  radiated  by  the  sun  to  the  earth,  if  suffered  to 
accumulate,  would  quickly  destroy  the  present  constitution  of  our 
globe.*  This  evil  is  prevented  by  the  radiation  of  heat  by  the 
earth  to  the  heavens,  during  the  night,  when  it  receives  from  them 
little  or  no  heat  in  return.  But  through  the  wise  economy  of 
means,  which  is  witnessed  in  all  the  operations  of  nature,  the  pre- 
vention of  this  evil  is  made  the  source  of  great  positive  good.  For 
the  surface  of  the  earth,  having  thus  become  colder  than  the 
neighbouring  air,  condenses  a  part  of  the  watery  vapour  of  the 
atmosphere  into  dew,  the  utility  of  which  is  too  manifest  to  require 
my  speaking  of  it.  I  may  remark,  however,  that  this  fluid  appears 
chiefly  where  it  is  most  wanted,  on  herbage  and  low  plants,  avoid- 
ing, in  great  measure,  rocks,  bare  earth,  and  considerable  masses  of 
water.t  Its  production,  too,  by  another  wise  arrangement,  tends  to 
prevent  the  injury  that  might  arise  from  its  own  cause ;  since  the 
precipitation  of  water  upon  the  tender  parts  of  plants,  must  lessen 
the  cold  in  them  which  ocsasions  it.  I  shall  observe  in  the  last 

*  Count  Rumford  says ;  "  May  it  not  be  by  the  action  of  these  (frigorific) 
rays,  that  our  planet  is  cooled  continually,  and  enabled  to  preserve  the  same 
mean  temperature  for  ages,  notwithstanding  the  immense  quantities  of  heat  that 
are  generated  at  its  surface,  by  the  continual  action  of  the  solar  rays  1"  Phil. 
Trans.  1804,  p.  181. 

f  I  have  no  direct  observations  for  the  foundation  of  this  assertion  concerning 
considerable  masses  of  water.  But,  I  hold  it,  notwithstanding,  to  be  just; 
because,  as  soon  as  the  surface  of  the  water  is  in  the  least  cooled  by  radiation, 
the  particleSy  composing  it  must  fall  downwards,  from  their  increased  gravity, 
and  be  replaced  by  others  that  are  warmer.  The  whole  mass,  therefore,  can 
never,  in  the  course  of  a  single  night,  be  sufficiently  cool  to  condense  into  dew 
any  great  quantity  of  the  watery  vapour  of  the  atmosphere.  Besides;  I 
have  found,  that  even  a  small  mass  of  water,  as  will  be  more  particularly  men- 
tioned in  the  last  part  of  this  essay,  sometimes  acquires  no  weight  from  the 
reception  of  dew,  in  the  space  of  a  whole  night  favourable  to  the  formation  of 
that  fluid. 


THE  THEORY  OF  DEW.  39 

place,  that  the  appearance  of  dew  is  not  confined  to  any  one  part  of 
the  night,  but  occurs  during  its  whole  course,  from  means  the  most 
simple  and  efficacious.  For  after  one  part  of  the  air  has  deposited 
its  moisture  on  the  colder  surface  of  the  earth,  it  is  removed,  in 
consequence  of  that  agitation  in  the  atmosphere  which  exists  during 
its  stillest  states,  and  gives  place  to  another  having  its  quantity  of 
water  undiminished  ;  and  again,  as  the  night  proceeds,  a  portion  of 
air  which  had  before  deposited  all  the  moisture,  which  circumstan- 
ces at  that  time  permitted,  is  rendered  fit,  by  the  general  increase  of 
the  cold  of  the  atmosphere,  to  give  out  a  fresh  parcel,  when  it  comes 
anew  into  contact  with  the  ground. 

I.  The  first  fact  which  I  shall  here  attempt  to  explain,  is  the 
prevention,  either  wholly  or  in  part,  of  cold  from  radiation,  in  sub- 
stances on  the  ground,  by  the  interposition  of  any  solid  body  be- 
tween them  and  the  sky.     This  evidently  appears  to  arise  in  the 
following  manner.     The  lower  body  radiates  its  heat  upwards,  as  if 
no  other  intervened  between  it  and  the  sky  ;  but  the  loss,  which  it 
hence  suffers,  is  more  or  less  compensated  by  what  is  radiated  to  it 
from  the  body  above,  the  under  surface  of  which  possesses  always 
the  same,  or  very  nearly  the  same  temperature  as  the  air.     In  this 
way,  therefore,  is  to  be  accounted  for  the  warmth  of  the  substances, 
which  were  sheltered  from  the  sky  by  the  raised  board,  the  paste- 
board roof,  and  the  hollow  cylinders  of  earth  and  pasteboard.     In 
these  examples,  the  interposed  substances  cannot  be  supposed  to 
have  remitted  more  heat   than   they  received.     But  in  situations 
where  large  masses  of  bare  solid  matter  exist,  which  are  warmer 
than  the  atmosphere,  from  the  heat  of  the  preceding  day  or  other 
causes,  a  greater  heat  will  be  received  by  the  exposed  body  than 
what  is  radiated  by  itself.     For  example,  it  seems  certain  to  me, 
that  the  houses  surrounding  Lincoln's-Inn  Fields  had  an  influence 
upon   my  thermometers,  during   my  experiment  there  at  night, 
beyond  what  arose  from  their  merely  returning  a  quantity  of  heat, 
equivalent  to  that  which   they  received    from  the  surface  of  the 
garden.     It  is  not,  however,  absolutely  requisite  that  a  body  should 
be  itself  exposed  to  the  sky  On  a  clear  and  calm  night,  in  order  to 
become  colder  than  the  atmosphere  ;  exposure  to  the  influence  of 
another  body,  so  situated,  is  sufficient  for  the  production  of  a  slight 
degree  of  this  effect.     Thus  I  have  always  found  wool  attached  to 
the  underside  of  my  raised  board,  on  such  a  night,  to  be  a  little  colder 
than  the  air  ;  and  it  has  appeared  to  me  a  sufficient  reason  for  the 
fact,  that  the  wool  in  this  situation  was  in  some  degree  exposed  to 
the  influence  of  grass,  which  had  become  considerably  colder  than 
the  atmosphere  by  radiating  its  heat  to  the  sky, 

II.  No  direct  experiments  can  be  made  to  ascertain  the  manner 
in  which  clouds  prevent,  or  occasion  to  be  small,  the  appearance  of 
a  cold  at  night,  upon  the  surface  of  the  earth,  greater  than  that  of  the 
atmosphere  ;  but  it  may,  I  think,  be  firmly  concluded,  from  what 
has  been  said  in  the  preceding  article,  that  they  produce  this  effect, 
almost  entirely,  by  radiating  heat  to  the  earth,  in  return  for  that 


40  ESSAY  ON  DEW, 

which  they  intercept  in  its  progress  from  the  earth  towards  the 
heavens.  For  although,  upon  the  sky  becoming  suddenly  cloudy 
during  a  calm  night,  a  naked  thermometer,  suspended  in  the  air, 
commonly  rises  2  or  3  degrees,  little  of  this  rise  is  to  be  attributed 
to  the  heat  evolved  by  the  condensation  of  watery  vapour  in  the 
atmosphere,  as  was  supposed  by  Mr.  Wilson  ;*  since,  in  consequence 
of  the  ceasing  of  that  part  of  the  cold  indicated  by  the  thermometer, 
which  was  owing  to  its  own  radiation  to  a  clear  sky,  the  tempera- 
ture of  the  atmosphere  may  seem  to  increase  2°,  or  more,  notwith- 
standing that  it  has  received  no  real  addition.  Besides  ;  the  heat 
which  is  extricated  by  the  condensation  of  vapour,  during  the  for- 
mation of  a  cloud,  must  soon  be  dissipated  ;  whereas  the  effect  of 
greatly  lessening,  or  preventing  altogether,  the  appearance  of  a  supe- 
rior cold  on  the  earth  to  that  of  the  air,  will  be  produced  by  a  cloudy 
sky,  during  the  whole  of  a  long  night. 

Dense  clouds,  near  the  earth,  must  possess  the  same  heat  as  the 
lower  atmosphere,  and  will  therefore  send  to  the  earth,  as  much,  or 
nearly  as  much  heat  as  they  receive  from  it  by  radiation.  But 
similarly  dense  clouds,  if  very  high,  though  they  equally  incercept 
the  communication  of  the  earth  with  the  sky,  yet  being,  from  their 
elevated  situation,  colder  than  the  earth,  will  radiate  to  it  less  heat 
than  they  receive  from  it,  and  may,  consequently,  admit  of  bodies 
on  its  surface  becoming  several  degrees  colder  than  the  air.  In  the 
first  part  of  this  Essay,  an  example  was  given  of  a  body  on  the 
ground  becoming  at  night  5°  colder  than  the  air,  though  the  whole 
sky  was  thickly  covered  with  high  clouds,  t 

Islands,  and  parts  of  continents  close  to  the  sea,  being,  by  their 
situation,  subject  to  a  cloudy  sky,  will,  from  the  smaller  quantity  of 
heat  lost  by  them  through  radiation  to  the  heavens  at  night,  in  addi- 
tion to  the  reasons  commonly  assigned,  be  less  cold  in  winter  than 
countries  considerably  distant  from  any  ocean. 

III.  Fogs,  like  clouds,  will  arrest  heat,  which  is  radiated  upwards 

*Edin.  Phil.  Trans.  I.  157. 

•{•  Mr.  Prevost  of  Geneva,  in  his  work  on  Radiant  Heat,  p.  382,  has  already  in 
this  way,  conjecturally  accounted  for  the  effect  of  clouds,  in  diminishing  at  night 
the  cold  of  the  atmosphere,  and  of  the  surface  of  the  earth  ;  but  he  seems  not  to 
have  known,  that  their  effect  on  the  temperature  of  the  latter  is  much  greater 
than  that  which  they  produce  upon  the  air.  My  explanation  of  this  influence  of 
clouds,  on  the  temperature  of  the  surface  of  the  earth,  during  the  night,  is  a  di- 
rect consequence  from  the  facts,  which  I  had  observed  respecting  the  prevention 
of  cold  on  the  ground  from  radiation,  by  the  interposition  of  solid  bodies  between 
it  and  the  heavens,  and  occurred  to  me  in  1812.  Mr.  Prevost's  work,  indeed, 
was  published  in  1809  ;  but  I  did  not  see  it  before  the  summer  of  1813 ;  when  it 
was  lent  to  me  by  his  relation  Dr.  Marcet  of  London,  who  at  the  same  time  said, 
that  he  believed  there  was  no  other  copy  of  it  in  Great  Britain,  except  one,  which 
had  been  sent  by  himself  to  Edinburgh. 

Note  to  second  edition.]  I  did  not  know,  until  after  the  first  edition  of  this 
Essay  was  printed,  that  Mr.  Prevost  had  published  his  opinion  on  the  effect  of 
clouds  in  preventing  the  occurrence  of  cold  at  night  in  the  atmosphere,  and  upon 
the  surface  of  the  earth,  as  early  as  1792,  in  a  work  entitled  « Recherches  sur  la 
Chaleur.' 


THE  THEORY  OF  DEW.  41 

by  the  earth,  and,  if  they  be  very  dense,  and  of  considerable  per- 
pendicular extent,  may  remit  to  it  as  much  as  they  receive.  Ac- 
cordingly, Mr.  Wilson  found  no  difference  at  night,  in  very  foggy 
weather,  between  the  temperature  of  the  surface  of  snow,  and  that 
of  the  air.  Several  observations  by  myself  tend  to  confirm  that  of 
Mr.  Wilson.  An  instance,  however,  as  was  formerly  said,  occurred 
to  me  of  a  difference  at  night  of  9°  between  the  temperatures  of 
grass  crusted  over  with  hoarfrost,  and  of  air,  during  a  very  dense 
fog.  A  fact,  remarked  by  Mr.  Leslie,  respecting  fogs,  serves  to 
explain  this  apparent  anomaly.  For  it  was  found  by  that  philoso- 
pher,* from  experiments  made  with  his  photometer,  that  in  mists 
and  low  fogs  the  diminution  of  the  sun's  heat  is  small,  when  com- 
pared with  what  occurs  when  the  sky  is  obscured  by  a  dense  body 
of  clouds  ;  and  it  will,  I  presume,  be  readily  granted,  that  the  same 
state  of  the  atmosphere,  which  allows  the  heat  of  the  sun  to  pass 
copiously,  will  also  give  a  ready  transit  to  heat  radiated  by  the  earth. 
Now  there  are  several  reasons  for  believing,  that  the  fog  during 
which  grass  was  9°  colder  than  the  air,  did  not  ascend  far  above  the 
ground.  1.  The  barometer  had  been  falling  for  some  days  before, 
and  it  is  a  matter  of  common  observation,  that  great  fogs  seldom 
occur,  except  it  be  high.  2.  On  the  day  preceding  the  observation, 
the  air,  after  having  been  extremely  foggy  for  nearly  a  week,  had 
become  clear  enough  to  allow  the  sun's  being  distinctly  seen  during 
the  whole  of  the  afternoon,  though  there  was  still  a  sufficient  ob- 
scurity in  the  lowermost  parts  of  the  atmosphere,  to  obstruct  consid- 
erably the  view  of  objects  on  the  ground  and  very  near  to  it.  3.  On 
the  day  following  the  observation,  the  fog  was  again  much  less  ; 
on  the  next  it  disappeared,  and  was  succeeded  by  snow.  It  is  to  be 
mentioned  likewise,  that  on  the  evening  in  question  the  state  of  the 
grass,  which  was  the  subject  of  experiment,  was  unusually  favoura- 
ble to  the  production  of  cold  ;  since,  contrary  to  general  experience, 
it  was  as  cold  as  swandown.  If,  then,  the  latter  substance,  from  the 
much  greater  regularity  of  the  appearances  exhibited  by  it,  be  taken 
as  the  standard,  by  which  the  occurrences  of  different  nights  are  to 
be  compared  together,  it  will  follow  that  the  fog  of  which  I  am 
speaking,  though  it  did  not  prevent,  must  have  lessened  the  produc- 
tion of  cold  from  radiation.  For,  on  the  preceding  evening,  when 
there  was  little  fog,  the  atmosphere  being  equally  still  on  both,  the 
difference  between  swandown  and  the  air  was  12°;  and  on  another, 
a  fortnight  after,  the  difference  at  the  same  place  of  observation 
between  thermometers  in  the  same  situations,  was  14^°,  the  air 
being  now  free  from  fog.  If  the  atmosphere  had  been  as  still  on 
this  as  on  the  former  evenings,  a  greater  difference  would  doubtless 
have  been  seen.  I  conclude,  therefore,  that  fogs  do  not  in  any  in- 
stance furnish  a  real  exception  to  the  general  rule,  that  whatever 
exists  in  the  atmosphere,  capable  of  stopping  or  impeding  the  pas- 
sage of  radiant  heat,  will  prevent  or  lessen  the  appearance  at  night 

*  On  Heat  and  Moisture,  p  57. 
OCT.  1838.—  T  5 


42  ESSAY  ON  DEW. 

of  a  cold  on  the  surface  of  the  earth  greater  than  that  of  the  neigh- 
bouring air. 

It  follows  also,  from  what  has  been  said  in  this  article,  that  the 
water  deposited  upon  the  earth,  during  a  fog  at  night,  may  some- 
times be  derived  from  two  different  sources,  one  of  which  is  a  pre- 
cipitation of  moisture  from  a  considerable  part  of  the  atmosphere, 
in  consequence  of  its  general  cold  ;  the  other,  a  real  formation  of 
dew  from  the  condensation,  by  means  of  the  superficial  cold  of  the 
ground,  of  the  moisture  of  that  portion  of  the  air,  which  comes  in 
contact  with  it.  In  such  a  state  of  things,  all  bodies  will  become 
moist,  but  those  especially  which  most  readily  attract  dew  in  clear 
weather.*  I  have  had  no  opportunity,  however,  of  trying  this 
conclusion  by  the  test  of  observation  since  it  occurred  to  me. 

IV.  When  bodies  become  cold  from  radiation,  the  degree  of  effect 
observed  must  depend  not  only  on  their  radiating  power,  but  in 
part  also  on  the  greater  or  less  ease  with  which  they  can  derive 
heat,  by  conduction,  from  warmer  substances  in  contact  with  them. 
Thus  grass  on  a  clear  and  still  night  was  constantly  colder,  some- 
times very  much  colder  than  the  gravel  walk,  though  a  small 
quantity  of  sand  placed  upon  grass  was  always  nearly  as  cold  as 
this  substance.  In  this  case,  the  difference  in  temperature  between 
the  gravel  walk  and  sand,  evidently  depended  on  the  different 
quantities  of  heat  which  they  received  from  the  parts  beneath.  A 
like  reason  is  to  be  given  for  dew  appearing  in  greater  quantity  on 
shavings  of  wood  than  on  the  same  substance  in  a  more  dense  and 
compact  form  ;  and  for  filamentous  and  downy  substances  becoming 
colder  than  all  others,  even  than  lampblack,  which  is  placed  by  Mr. 
Leslie,  at  the  head  of  the  best  solid  radiators  of  heat.  For  the 
lampblack  exposed  by  me  being  about  2  lines  in  depth,  possessed,  in 
consequence,  a  fund  of  internal  heat,  which  would  more  readily  pass 
to  its  cold  surface,  than  the  heat  of  the  lower  parts  of  the  downy 
substances  would  to  their  upper  surface. 

This  subject  is  illustrated  by  the  following  experiment.  On  a 
dewy  evening  I  depressed  into  a  soft  garden  mould  a  drinking  glass 
having  a  thick  flat  bottom,  until  its  brim  was  upon  a  level  with  the 
surrounding  earth,  and  at  the  same  time  placed  a  similar  vessel  with 
its  cavity  also  towards  the  sky,  on  the  surface  of  the  mould.  In  the 
morning  the  inside  of  the  depressed  glass  was  entirely  dry,  while 
that  of  the  other  was  dewed.  I  then  applied  the  bulb  of  a  small 
thermometer  to  the  inside  of  the  bottom  of  each  vessel,  on  which  I 
found  the  heat  of  that  part  of  the  depressed  one  to  be  56°,  but  of  the 
same  part  of  that  which  stood  on  the  mould  only  49i°.  At  this 
time  the  temperature  of  the  air  was  53°.  The  cause,  therefore,  was 
evident,  both  of  the  witness  of  the  first  vessel  and  of  the  dryness  of 
the  second. 

From  this  source  also  is  to  be  derived  the  reason,  why  the  promi- 

*  The  moisture  observed  at  night  by  Musschenbroek  in  Holland,  and  called 
by  him  dew,  appears  to  me  to  have  been  of  this  kind.  See  this  Essay,  p.  6. 


THE  THEORY  OF  DEW.  43 

nent  parts  of  various  bodies  were  observed  by  Mr.  Wilson  to  be 
crusted  with  hoarfrost,  while  their  more  retired  and  massy  parts 
were  free  from  it.* 

V.  Bodies  exposed  in  a  clear  night  to  the  sky,  must  radiate  as 
much  heat  to  it  during  the  prevalence  of  wind  as  they  do  if  the  air 
were  altogether  still.     But  in  the  former  case  little  or  no  cold  will 
be  observed  upon  them  above  that  of  the  atmosphere,  as  the  frequent 
application  of  warm  air  must  quickly  return  a  heat  equal,  or  nearly 
so,  to  that  which  they  had  lost  by  radiation.     A  slight  agitation  of 
the  air  is  sufficient  to  produce  some  effect  of  this  kind ;  though,  as 
has  already  been  said,  such  an  agitation  when  the  air  is  very  preg- 
nant with  moisture,  will  render  greater  the  quantity  of  dew  ;  one 
requisite  for  a  considerable  production  of  this  fluid  being  more  in- 
creased by  it,  than  another  is  diminished. 

VI.  A  small  body,  as  a  thermometer,  suspended  in  the  air,  will 
even  in  the  calmest  night  exhibit  but  little  cold  from  radiation,  since 
it  is  continually  exposed  to  the  application  of  fresh  parcels  of  warmer 
air,  both  from  the  progressive  motion  of  this  fluid  and  from   the 
downward  motion  produced  in  it  by  the  superior  gravity  of  such 
portions  as  have  been  cooled  by  contact  with  the  suspended   body. 
On  the  other  hand,  a  thermometer  upon  a  board,  raised  above  the 
earth  and  possessing  a  surface  of  several  square  yards,  will  have  its 
cold  from  radiation  much  less  diminished  than  the  former,  as  it  is 
exposed  to  no  loss  from  a  downward  motion  of  the  air,  and  as  the  air 
which  approaches  it  horizontally  must  almost  always  have  had  its 
temperature  previously  lowered  by  passing  over  another  part  of  the 
board.     The  reason  then  of  the  lee  side  of  the  raised  board   being 
often  colder  than  the  windward  is  obvious. 

VII.  There  is  a  remark  by  Theophrastus.t  which  has  been  con- 
firmed by  other  writers,  that  the  hurtful  effects  of  cold  occur  chiefly 
in  hollow  places.     If  this  be  restricted  to  what  happens  on  serene 
and  calm  nights,  and  it  does  not,  I  believe,  hold  true  in  any  other 
circumstances,  two  reasons  from  different  sources  are  to  be  assigned 
for  it.     The  first  is,  that  the  air  being  stiller  in  such  a  situation  than 
in  any  other,  the  cold  from  radiation  in  the  bodies  which  it  contains 
will  be  less  diminished  by  renewed  applications  of  warmer  air;  the 
second,  that  from  the  longer  continuance  of  the  same  air  in  contact 
with  the  ground,  in  depressed  places  than  in  others,  less  dew  will  be 
deposited,  and  therefore  less  heat  extricated  during  its  formation. 
It  will  be  seen  in  the  last  part  of  this  Essay,  that  in  the  East  Indies 
depressions  in  the  earth  are  artificially  made,  for  the  purpose  of  in- 
creasing the  cold,  which  appears  in  serene  nights.     On  this  Subject, 
however,  it  is  to  be  observed,  that  if  the  depressed  or  hollow  places 
be  deep,  in  proportion  to  their  horizontal  extent,  a  contrary  effect 
must  follow ;  as  a  case  will  occur  more  or  less  similar  to  that  which 
existed  in  some  experiments  formerly  related  by  me,  in  which  a 
small  portion  of  grass  was  surrounded  by  a  hollow  cylinder. 

*  Paper  in  Phil.  Trans.  1780.  f  Lib-  v«  c-  xvi- 


44  ESSAY  ON  DEW. 

VIII.  An  observation  closely  connected  with  the  preceding, 
namely,  that  in  clear  and  still  nights  frosts  are  less  severe  upon 
hills  than  in  neighbouring  plains,*  has  excited  more  attention, 
chiefly  from  its  contradicting  what  is  commonly  regarded  an  esta- 
blished -fact,  that  the  cold  of  the  atmosphere  always  increases 
with  the  distance  from  the  earth.  This  inferior  cold  of  hills  is 
evidently  a  circumstance  of  the  same  kind  with  that  ascertained  by 
Mr.  Pictet  and  Mr.  Six  respecting  the  increasing  warmth,  in  clear 
and  calm  nights  at  all  seasons  of  the  year,  of  the  different  strata  of 
the  atmosphere,  in  proportion  as  these  are  more  elevated  above  the 
earth.  As  the  greater  cold  of  the  lower  air  is  the  less  complicated 
fact,  I  shall  attempt  to  explain  it  in  the  first  place.  Mr.  Pictet, 
indeed,  furnishes  an  explanation  himself,  by  ascribing  it  to  the  evap- 
oration of  moisture  from  the  ground.  But  to  show  that  this  is  not 
just,  it  need  only  be  mentioned,  that  the  appearance  never  occurs 
in  any  considerable  degree,  except  upon  such  nights  as  are  attended 
with  some  dew,  and  that  its  great  degrees  are  commonly  attended 
with  a  copious  formation  of  that  fluid  ;  since  it  cannot  be  thought 
that  the  same  stratum  of  air  will  deposit  moisture  on  the  ground, 
from  an  insufficiency  of  heat,  at  the  very  time  it  is  receiving  moisture 
from  the  ground,  in  the  state  of  pellucid  vapour,  as  this  presupposes 
that  it  is  not  yet  replete  with  water. 

Our  atmosphere  has  been  very  generally  regarded,  as  incapable 
of  being  heated  directly  by  the  rays  of  the  sun,  principally  because 
these  give  no  heat  to  any  particular  portion  of  it,  in  which  they 
are  brought  to  a  focus.  I  do  not  know,  whether  this  experiment 
was  ever  made  with  all  the  accuracy  of  which  it  is  susceptible  ;  but 
granting  that  it  has  been  thus  made,  my  opinion  is,  notwithstanding, 
that  no  reliance  can  be  placed  in  it.  For  as  air,  if  heated  at  all  by 
concentrated  sunbeams,  must  be  heated  by  them  in  a  very  slight 
degree  during  the  time  that  their  focus  may  be  looked  upon  as 
stationary,  otherwise  the  present  question  would  not  have  arisen,  it 
is  necessary  for  conducting  the  experiment  properly,  that  during 
the  whole  of  it,  the  same  individual  small  portion  of  air  shall  con- 
stantly receive  that  focus  ;  but  this,  for  various  manifest  reasons, 
cannot  possibly  happen.  Viewing  therefore  the  argument  founded 
upon  this  experiment  as  without  force,  I  shall  now  offer  several 
considerations,  which  seem  to  prove,  that  air  is  actually  heated  by 
the  sunbeams  which  enter  it. 

1.  Air  both  reflects  and  refracts  light,  and  all  other  bodies,  as  far 
as  I  know,  acquire  heat,  while  they  act  thus  on  the  light  of  the  sun. 

2.  Air  suffocates  or  absorbs  the  sun's  light,  which  it  cannot  be 
supposed  to  do  without  increasing  in  temperature. 

3.  If  air,  considered  as  an  uniform  fluid,  were  even  incapable  of 
gaining  heat  directly  from  the  sun?s  rays,  heat  would  be  communi- 
cated by  them  to  it,  through  the  intervention  of  the  innumerable 

*  Theophrastus  also  remarks,  that  it  freezes  less  on  [hills  than  on  plains,  but 
without  mentioning,  that  this  happens  only  on  calm  and  serene  nights.  Lib.  v. 

C.  XX. 


THE  THEORY  OF  DEW.  45 

particles  of  solid  matter,  which  the  trivial  experiment  of  receiving 
a  sunbeam  into  a  darkened  room  shows  to  be  present  in  the  atmo- 
sphere. Should  it  be  said,  that  this  appearance  may  occur  only  in 
the  neighbourhood  of  the  earth,  from  the  accidental  admixture  of 
solid  matter  raised  from  its  surface  by  winds,  or  in  any  other 
way,  the  answer  is  that,  as  my  inquiry  is  concerning  the  existence 
of  a  certain  condition  of  the  atmosphere,  it  matters  not  how  this 
originates.  Nothing  more  can  be  demanded  than  that  it  should 
always  be  found,  which  I  believe  to  be  the  case  ;  since,  if  I  can  trust 
my  memory  with  respect  to  what  took  place  many  years  ago,  I 
should  say  that  such  particles  are  to  be  seen,  by  means  of  the  sun's 
light  in  the  air,  over  the  middle  of  the  Atlantic  ocean.  These  par- 
ticles then,'  must  receive  heat  from  the  sunbeams,  which  impinge 
upon  them,  and  this  they  will  communicate  to  the  contiguous  pel- 
lucid air. 

4.  Unless  it  be  admitted,  that  the  atmosphere  is  capable  of  inter- 
cepting part  of  the  heat  which  is  radiated  into  it  by  the  sun,  and  of 
converting  this  into  heat  of  temperature,  I  deem  it  impossible  to 
find  a  sufficient  reason  for  the  great  warmth  which  exists  after  a 
long  calm,  in  air  incumbent  upon  the  Atlantic  and  Pacific  oceans,  at 
the  distance  of  a  thousand  miles  or  more  from  any  considerable  body 
of  land.  It  cannot  be  derived  from  the  neighbouring  water,  since 
this  is  colder  than  the  lower  atmosphere  ;  and  no  one  will  suppose 
it  to  be  the  same  heat  which  the  air  had  acquired  from  the  last 
continent  it  had  passed  over  many  days  before.  But  if  even  this 
were  supposed,  another  difficulty  would  remain  to  be  removed, 
which  is,  that  during  the  whole  of  the  calm  the  air  is  cooled  every 
night,  and  again  becomes  warm  in  the  day.* 

Should  what  has  .been  said  be  thought  sufficient  to  establish,  that 
the  air  arrests  part  of  the  sun's  heat,  which  is  radiated  into  it  bound 
up  with  light,  two  consequences  must  also  be  allowed.  The  first  is, 
that  air  will  exert  a  greater  power  of  the  same  kind  upon  heat  ra- 
diated into  it  without  light,  since  the  sun's  heatpasses  instantaneously 
through  many  bodies,  which  refuse  a  similar  way  to  heat  radiated 
by  terrestrial  substances  ;  the  other,  that  air  must  be  as  capable  of 
becoming  cold  by  radiating  its  own  heat,t  as  of  becoming  warm 
from  heat  radiated  into  it,  as  these  two  properties  are  uniformly 
observed  to  exist  together,  and  to  be  proportional  to  each  other. 
The  truth  of  the  latter  conclusion  may  also  be  inferred  from  this 
fact,  that  in  still  and  calm  weather  the  heat  of  the  air,  a  few  feet 
above  the  earth,  will  sometimes  decrease,  even  in  this  country, 
18  or  20  degrees  between  sunset  and  sunrise,  though  no  change 
of  wind  has  in  the  meantime  occurred  ;  for  the  inconsiderable 
conducting  power,  which  air  is  now  known  to  possess,  will  per- 

*  One  reason  is  hence  apparent  for  the  great  coldness  of  the  high  regions  of 
the  atmosphere  ;  since  the  air  in  them  must  be  less  fit  than  that  of  the  lower 
strata  to  arrest  heat  which  is  radiated  into  it. 

|  M.  Prevost  says  :  "On  peut  supposer  queles molecules  de  1'airrayonnent." 
Du  Calorique  Rayonnant,  p.  24. 


46  ESSAY  ON  DEW. 

mit  only  a  small  part  of  this  diminution  to  arise  from  heat  passing, 
by  means  of  that  power,  from  the  atmosphere  to  the  colder  earth. 
Mr.  Leslie,*  indeed,  ascribes  this  effect  to  the  descent  of  cold  air 
from  the  higher  regions  of  the  atmosphere  ;  but  if  this  were  just, 
a  less  cold  ought  to  be  found,  on  a  clear  and  still  night,  in  the 
lower  than  in  the  higher  strata,  which  is  contrary  to  the  uniform 
results  of  numerous  experiments  by  Mr.  Pictet  and  Mr.  Six.  Winds, 
too,  which  produce  such  a  mixture,  always  lessen  the  nocturnal 
decrease  of  temperature  in  the  lowermost  part  of  the  atmosphere. 

Having  thus  shown,  that  air  is  capable,  both  of  absorbing  heat 
which  is  radiated  into  it,  and  of  radiating  heat  which  had  before 
formed  a  part  of  its  temperature,  I  proceed  to  apply  the  knowledge 
of  these  facts,  to  the  explanation  of  the  phenomenon  observed  by 
Mr.  Pictet  and  Mr.  Six. 

This  phenomenon  occurs  on  those  nights  only,  which  permit 
bodies,  on  the  surface  of  the  earth,  to  become  cold  by  radiating  their 
heat  to  the  heavens.  On  other  nights,  when  bodies,  thus  situated, 
were  not  colder  than  the  air,  I  have  observed  the  atmosphere,  within 
the  limits  of  9  feet  from  the  ground,  the  boundary  of  my  own  ex- 
periments, to  decrease  a  little  in  temperature  as  the  distance  from 
the  earth  increased.  Mr.  Six  likewise  found,  that,  on  cloudy  nights, 
the  air  was  sometimes  colder  220  feet  above  the  ground  than  at  the 
distance  of  9  feet  from  it.  When,  therefore,  the  earth  has  become 
colder,  from  radiation,  than  the  neighbouring  air,  in  consequence 
of  the  latter  having,  by  reason  of  its  small  radiating  power,  emitted 
a  less  proportion  of  its  heat  to  the  heavens,  the  warmer  air  must 
radiate  a  part  of  its  heat  to  the  earth  without  receiving  a  full  com- 
pensation, and  will  therefore  become  colder  than  it  otherwise 
would  have  been.  In  proportion  too  as  the  air  is  nearer  to  the 
earth  must  the  cold  of  the  former  from  this  cause  be  the  greater. 
My  own  conception  of  this  matter  is  facilitated,t  by  contemplating 
the  occurrence  of  an  opposite  effect,  when  the  earth  is  warmer  than 
the  air.  Let  it  be  supposed  then,  that  while  the  earth,  in  this  state, 
radiates  upwards  a  quantity  of  heat,  a  foot  in  depth  of  the  incumbent 
air  is  capable  of  stopping  a  1000th  of  what  it  hence  receives,  and  of 
converting  it  into  heat  of  temperature.  The  consequence  must  be 
that  the  next  foot,  from  receiving  only  999  parts  of  what  has  been 
emitted  by  the  earth,  will  not  be  so  much  heated  as  the  first  foot, 
though  it  should  absorb  the  same  proportional  quantity  of  what 
enters  it.  In  this  way,  every  successive  foot  will  acquire  a  less 
quantity  of  heat  than  the  preceding,  and  a  state  of  the  atmosphere 
be  produced,  like  to  that  which  is  actually  observed  in  a  calm  and 
sunny  day.  In  the  day,  however,  the  phenomena,  from  the  heating 
of  air  by  rays  from  the  earth,  are  somewhat  confused  by  the  warmed 
portions  rising  upwards,  and  mixing  with  what  is  colder  ;  whereas, 
at  night,  the  air,  which  has  been  cooled  by  radiating  heat  to  the 

*  On  Heat  and  Moisture,  p.  11,  and  132. 

|  The  same  facility  is  afforded  by  considering  cold  as  a  body. 


THE  THEORY  OF  DEW.  47 

earth,  is  rendered,  by  an  increase  of  gravity,  the  more  fit  to  retain 
its  low  position.  I  have  here,  for  the  sake  of  simplifying  the  argu- 
ment, taken  no  notice  of  the  cooling  of  any  considerable  mass  of  the 
air,  in  consequence  of  the  actual  contact  of  its  lowermost  stratum 
with  the  earth,  or  by  the  conduction  of  the  temperature  of  one  por- 
tion of  it  to  another.  But,  in  a  calm  state  of  the  atmosphere  these 
effects  must  be  inconsiderable,  though  it  appears  to  me  impossible, 
in  the  present  state  of  our  knowledge  to  determine  them  with  any 
precision. 

According  to  the  view  which  has  been  given  by  me  of  this  subject, 
the  heat  of  the  air,  in  a  clear  and  calm  night,  ought  to  increase, 
within  the  limits  of  the  phenomenon,  in  some  decreasing  geometrical 
ratio,  as  the  atmosphere  ascends  ;  and  this  conclusion  is  so  far  con- 
firmed by  the  observations  of  Mr.  Pictet  and  Mr.  Six  taken  together, 
that  the  increase  of  temperature  is  found  to  be  greater  in  a  given 
space  very  near  to  the  earth  than  in  an  equal  space  more  remote 
from  it. 

To  return  to  the  immediate  object  of  this  article,  the  fact  is  cer- 
tain, whatever  may  be  thought  of  my  explanation  of  it,  that  in 
every  clear  and  still  night  the  air  near  to  the  earth  is  colder  than 
that  which  is  more  distant  from  it,  to  the  height  at  least  of  220  feet, 
this  being  the  greatest  to  which  Mr.  Six's  experiments  relate.  If 
then  a  hill  be  supposed  to  rise  from  a  plain,  to  the  height  of  220 
feet,  having  upon  its  summit  a  small  flat  surface  covered  with  grass; 
and  if  the  atmosphere,  during  a  calm  and  serene  night,  be  admitted 
to  be  10°  warmer  there  than  it  is  near  the  surface  of  the  low  ground, 
which  is  a  less  difference,  according  to  the  observations  of  Mr.  Six, 
than  what  sometimes  occurs  in  such  circumstances,  it  is  manifest 
that,  should  both  the  grass  upon  the  hill  and  that  upon  the  plain 
acquire  a  cold  of  10°  degrees  by  radiation,  the  former  will,  notwith- 
standing, be  10°  warmer  than  the  latter. 

But  the  equality  here  supposed  to  be  in  the  cold  acquired  by 
grass,  in  two  such  situations,  can  seldom  exist.  For  according  to 
an  observation  made  by  Aristotle,*  and  since  frequently  repeated, 
the  air  of  high  places  is  much  more  agitated  than  that  upon  low 
ground.  The  frequent  renewal,  therefore,  from  this  cause,  of  the 
air  in  contact  with  the  grass  on  the  hill,  will  prevent  it  from  ever 
becoming  much  colder  than  the  general  mass  of  the  atmosphere  at 
the  same  height.  Consequently,  any  diminution  in  this  way  of  the 
10°  of  cold  formerly  supposed  to  occur  there  from  radiation,  must 
be  added  to  the  difference  of  temperature  in  the  grass  in  the  two 
situations. 

What  has  hitherto  been  said  refers  only  to  the  occurrences  on  the 
very  summit  of  the  hill.  With  respect  to  its  sides,  these  can  be 
only  a  little  colder  than  the  atmosphere  upon  a  level  with  them, 
even  in  its  calmest  state.  For,  in  the  first  place,  they  do  not  enjoy 
the  full  aspect  of  the  sky  j  and,  in  the  second,  the  air  which  is 

*  Meteor,  lib.  l.c,  x. 


48  ESSAY  ON  DEW. 

cooled  by  contact  with  them,  will,  from  its  increased  gravity,  slide 
down  their  declivity,  and  thus  make  room  for  the  application  of  new 
and  warm  parcels  to  the  same  surface.  The  motion,  too,  thus 
excited  in  the  air,  near  to  the  sides  of  the  hill,  must  occasion  a 
motion  in  that  upon  the  summit,  which  may,  in  some  measure, 
account  for  the  last  mentioned  observation  of  Aristotle,  as  far  as 
relates  to  what  happens  in  a  clear  night. 

The  height  of  the  hill  in  this  example  has  been  supposed  to  be 
small,  to  make  it  accord  with  that  of  the  stations  whose  tempera- 
tures were  compared  by  Mr.  Six  with  the  heat  of  the  air  near  the 
ground.  But  observations  of  the  same  kind  will  apply  to  hills  of 
much  greater  elevation.  For  granting,  first,  that  the  air  at  the  height 
of  220  feet  is  never  more  than  10°  colder  than  that  near  to  the  earth, 
which  is  not  probable,  and  is  indeed  contradicted  by  some  of  Mr. 
Six's  observations  ;  and  again,  that  the  increase  of  the  air's  heat  in 
a  calm  and  serene  night,  ceases  precisely  at  the  greatest  height  to 
which  Mr.  Six  carried  his  observations,  which  is  also  improbable  ; 
still  a  reduction  to  the  extent  of  10°  in  the  temperature  of  the  air 
near  to  the  earth,  will  render  the  cold  of  this  low  portion  of  the 
atmosphere  greater  than  that  of  any  other  portion,  which  is  not 
more  than  2500  or  3000  feet  above  the  former,  if  the  estimate  be 
just,  which  makes  a  declension  in  the  heat  of  the  atmosphere  of  1° 
for  every  250  or  300  feet  of  its  height,  when  no  counteracting  cause 
exists. 

The  remarks,  however,  which  have  been  offered  on  the  greater 
warmth  of  hills  at  night,  in  a  certain  state  of  weather,  are  strictly 
applicable  to  those  only  which  are  insulated,  and  of  inconsider- 
able lateral  extent ;  and  it  is  upon  such  chiefly,  if  not  solely,  that 
this  phenomenon  has  been  observed.  The  superiority  of  the  cold 
of  a  low  plain,  from  radiation,  over  that  of  a  wide  expanse  of  hilly 
ground,  will,  for  obvious  reasons,  be  less  ;  and  no  superiority  of 
this  kind  will  probably  exist  in  the  former  situation,  when  the  high 
ground  is  not  only  extensive  but  flat  on  the  top,  forming  what  is 
called  a  table  land  ;  unless,  indeed,  which  seems  to  be  actually  the 
case,  the  air  of  such  an  elevated  country  should  be  commonly 
more  agitated  than  that  of  lower  places  equally  level. 

An  explanation  may  be  now  easily  given  of  an  observation  by 
Mr.  Jefferson  of  Virginia,*  which,  however,  had  also  been  made  by 
Aristotle,t  and  Plutarch, j  that  dew  is  much  less  copious  on  hills 
than  it  is  upon  plains.  For  allowing,  at  first,  the  surface  of  the 
ground  to  be  in  both  situations  equally  colder  than  the  air  which  is 
near  to  it  ;  still,  as  the  production  of  dew  must  be  in  proportion  to 
the  whole  depression  of  the  temperature  of  the  air  which  furnishes 
it,  below  what  its  heat  had  been  in  the  preceding  day,  and  as  one 
part  of  this  depression,  the  general  cooling  of  the  atmosphere,  is 
much  more  considerable  on  the  plain  than  on  the  hill,  moisture 
must  necessarily  be  deposited  more  copiously  in  the  former  than  in 

*  Notes  on  Virginia,  p,  132.    f  Meteor.  Lib.  1.  c.  x.    £  De  Primo  Frigido. 


THE  THEORY  OF  DEW.  49 

the  latter  place.  If  the  greater  agitation  of  the  atmosphere,  and 
the  less  quantity  of  moisture  during  clear  weather,  in  its  higher 
region  than  in  the  lower,  be  added,  it  may  readily  be  inferred  that 
dew  shall  sometimes  be  altogether  wanting  on  a  hill, though  abundant 
on  a  plain  at  its  foot,  agreeably  to  what  has  been  actually  observed 
by  Mr.  Jefferson. 

IX.  The  leaves  of  trees  often  remain  dry  throughout  the  night, 
while  those  of  grass  are  covered  with  dew.     As  this  is  a  similar 
fact  to  the  smallness  of  dew  on  hills,  I  shall  in  accounting  for  it  do 
little  more  than  enumerate  the  circumstances  on  which  it  depends. 

1.  The  atmosphere  is  several  degrees  warmer  near  the  upper  parts 
of  trees  on  dewy  nights  than  close  to  the  ground.  2.  The  air  in 
the  higher  situation  is  more  agitated  than  that  in  the  lower. 

3.  The  air  at  a  little  distance  from  the  ground,  from  being  nearer  to 
one  of  its  sources  of  moisture,  will  on  a  calm  evening  contain  more 
of   it   than    that   which  surrounds  the  leaves    of  elevated  trees. 

4.  Only  the  leaves  of  the  very  tops  of  trees  are  fully  exposed  to  the 
sky.     5.  The  declension  of  the  leaves  from  a   horizontal  position 
will   occasion  the  air,  which  has  been  cooled  by  them,  to  slide 
quickly  away,  and  be  succeeded  be  warmer  parcels.     6.  The  length 
of  the  branches  of  the  trees,  the  tenderness  of  their  twigs,  and  the 
pliancy  of  the  footstalks  of  their  leaves,  will  cause  in  the  leaves  an 
almost  perpetual  motion,  even  in  states  of  air  that  may  be  denomi- 
nated calm.     I  have  hence  frequently  heard,  during  the  stillness  of 
night,  a  rustling  noise  in  the  trees,  which  formed  one  of  the  bounda- 
ries of  the  ordinary  place  of  my  observations,  while  the  air  below 
seemed  without  motion. 

Nearly  in  the  same  manner  is  to  be  explained  why  shrubs  and 
bushes  also  receive  dew  more  readily  than  lofty  trees. 

X.  Bright  metals,  exposed  to  a  clear  sky  in  a  calm  night,  will  be 
less  dewed  on  their  upper  surface  than  other  solid  bodies  ;  since  of 
all  bodies  they  will,  in  such  a  situation,  lose  the  smallest  quantity 
of  heat  by  radiation  to  the  heavens,  at  the  same  time  that  they  are 
capable  of  receiving,  by  conduction,  at  least  as  much  heat  as  any 
others  from  the  atmosphere,  and  more  than  any  others  from  the 
warmer  solid  substances  which  they  happen  to  touch. 

If  the  exposed  pieces  of  metal  be  not  very  small,  another  reason 
will  contribute  somewhat  to  their  being  later  and  less  dewed  than 
other  solid  substances.  For  in  consequence  of  their  great  con- 
ducting power,  dew  cannot  form  upon  them,  unless  their  whole 
mass  be  sufficiently  cold  to  condense  the  watery  vapour  of  the 
atmosphere  ;  while  the  same  fluid  will  appear  on  a  bad  conductor  of 
heat,  though  the  parts  a  very  little  beneath  the  surface  are  warmer 
than  the  air.* 

*  I  hence  think  it  probable,  that  dew  will  sometimes  form  on  the  bulb  of  a 
thermometer,  before  the  mercury  in  it  is  cooled  below  the  temperature  of  the  air. 
It  seems  certain  to  me,  also,  that  dew  may  appear  upon  substances,  which,  from 
the  thinness  of  the  layer  of  matter  their  cold  is  confined  to,  will  produce  little  or 
no  sensible  effect  upon  a  thermometer  that  is  applied  to  them. 


-'•''.    '  :'•'      ''''•• 

50  ESSAY  ON  DEW. 

From  the  same  ready  passage  of  heat  from  one  part  of  a  metal  to 
another,  a  metallic  plate  suspended,  horizontally,  in  the  air  several 
feet  above  the  ground,  will  be  found  dewed  on  its  lower  side,  if  the 
upper  has  become  so  ;  while  the  lower  surface  of  other  bodies,  more 
attractive  of  dew,  but  worse  conductors  of  heat,  are  without  dew  in 
a  similar  situation. 

A  metal  placed  at  night  in  the  air,  near  to  the  ground,  is,  for  the 
most  part,  sufficiently  cold  to  condense,  on  its  underside,  the  vapour 
which  arises  from  the  warmer  earth  ;  though  its  upper  surface  may 
be  dry,  from  possessing  the  same,  or  almost  the  same  temperature, 
as  the  atmosphere  near  to  it. 

As  the  temperature  of  metals  is  never  much  below  that  of  the 
neighbouring  air,  a  slight  diminution  of  their  cold  from  radiation 
will  often  occasion  them  to  evaporate  the  dew,  which  they  had 
previously  acquired,  though  other  substances,  which  had  been  more 
cooled  by  radiation,  are  still  attracting  dew.  For  a  like  reason,  a 
metal,  which  has  been  purposely  wetted,  will  often  become  dry  at 
night,  while  other  substances  are  becoming  moist. 

A  substance  highly  attractive  of  dew,  such  as  wool,  if  laid  upon 
a  metal,  will  derive  heat  from  it,  and  will  therefore  acquire  less 
dew  than  an  equal  portion  of  the  same  substance  laid  upon  grass. 

A  large  metallic  plate  will  be  less  readily  dewed  while  lying  on 
grass,  than  if  it  were  placed  in  the  air,  though  only  a  few  inches 
above  the  grass  ;  because,  in  the  former  situation,  it  receives  freely, 
by  means  of  its  great  conducting  power,  heat  from  the  earth ;  where- 
as, when  placed  in  the  air,  it  powerfully  resists  by  another  property, 
possessed  in  a  great  degree  by  bright  metals,  the  entrance  of  heat 
radiated  towards  it  by  the  grass  beneath.  Besides  ;  the  grass  under 
the  metal  possesses  now  less  heat  than  when  this  substance  was  in 
contact  with  it,  partly  from  having  a  small  oblique  aspect  of  the 
sky,  and  partly  from  receiving  air,  which  has  been  cooled  by  pass- 
ing over  other  grass  fully  exposed  to  the  heavens. 

When  a  piece  of  metal,  having  closely  applied  to  its  under  surface 
a  substance  of  some  thickness,  which  attracts  dew  powerfully,  and 
therefore  imbibes  readily  heat  that  is  radiated  to  it,  is  exposed  to 
the  sky  at  night,  the  heat  supplied  by  the  attached  substance,  both 
from  its  own  original  store,  and  from  what  it  has  acquired  through 
the  radiation  of  the  ground  to  it  during  the  exposure,  will  enable 
this  piece  to  resist  longer  than  a  bare  piece  the  formation  of  dew, 
or  even  than  another  piece  which  has  only  a  thin  coat  of  matter  con- 
siderably attractive  of  dew  attached  to  its  underside.  The  experi- 
ment with  the  wooden  cross,  covered  with  gilt  paper,  affords  an 
example  of  the  latter  fact. 

A  very  small  metallic  plate,  suspended  in  the  air,  is  less  readily 
dewed  than  a  large  one  similarly  situated,  as  it  receives,  in  propor- 
tion to  its  size,  more  heat  from  the  atmosphere.  On  the  other 
hand,  a  very  small  plate  laid  upon  grass,  rendered  cold  by  radiation, 
will  be  sooner  dewed  than  a  larger  one  in  the  same  situation,  from 
presenting  a  greater  proportional  circumference  to  the  surrounding 


THE  THEORY  OF  DEW.  51 

grass,  and  therefore  losing  more  quickly  its  heat  by  conduction. 
It  will  he  also  sooner  dewed  than  another  very  small  plate  suspended 
in  the  air;  since  the  latter, like  other  small  bodies  similarly  placed, 
must  be  continually  acquiring  more  heat  than  the  former,  in  the 
manner  described  above  in  this  Essay.* 

A  piece  of  metal,  applied  to  different  portions  of  cold  grass  in 
succession,  will  sooner  become  cold  itself,  than  another  piece  which 
is  suffered  to  remain  constantly  upon  one  portion  of  the  same  grass, 
and  will  in  consequence  be  sooner  dewed. 

If  the  bare  side  of  a  piece  of  metalled  paper  be  exposed  to  a  clear 
and  calm  sky  at  night,  it  will  become  cold,  by  radiation,  and  receive, 
by  conduction,  the  heat  of  the  inferior  metallic  surface  ;  whence,  if 
this  surface  be  afterwards  made  the  upper  one,  it  will  sooner  acquire 
dew  than  a  similar  metallic  surface  which  has  been  exposed  to  the 
sky  during  the  whole  of  the  experiment. 

When  a  metal  covers,  in  part  only,  the  upper  surface  of  a  piece 
of  glass,  the  uncovered  portion  of  the  glass  quickly  becomes  cold  by 
radiation,  on  exposure  to  a  serene  sky  in  a  still  night,  and  then,  by 
deriving  to  itself  a  part  of  the  heat  of  the  metal,  occasions  this  body 
to  be  more  readily  dewed  than  if  the -whole  of  the  exposed  surface 
had  been  metallic.  In  this  experiment,  the  outer  edge  of  the  metallic 
surface,  from  being  nearest  to  the  colder  glass,  will  be  the  first  and 
the  most  dewed,  while  the  parts  of  the  uncovered  glass,  which  are 
contiguous  to  the  warmer  mettl,  will  be  the  last  and  the  least  dewed, 
of  their  respective  substances. 

A  piece  of  glass,  covered  on  one  side  with  a  metal,  being  placed 
on  grass,  with  this  side  down,  its  upper  surface  attracts  dew  as 
readily  as  if  no  metal  were  attached  to  it ;  since  the  metal  in  this 
situation  has  no  power  to  lessen  the  radiation  of  heat  from  the  upper 
surface  of  the  glass.  I  conclude,  however,  from  general  principles, 
for  I  have  not  made  the  trial,  that  if  the  same  piece  of  glass,  having 
its  metallic  side  still  undermost,  were  raised  in  the  air  a  little  above 
the  grass,  it  would  be  more  readily  dewed  on  its  upper  surface  than 
if  it  had  been  without  a  metallic  coating  on  the  lower,  as  this  coating 
must  resist;  the  introduction  of  heat  radiated  by  the  warmer  grass, 
and  thus  preserve  nearly  undiminished  the  cold  acquired  from  radi- 
ation of  heat  to  the  sky,  by  the  bare  upper  surface. 

The  preceding  remarks  apply  to  the  whole  class  of  metals  ;  but 
the  discoveries  of  Mr.  Leslie,  respecting  the  difference  in  the  capa- 
cities of  these  bodies  to  radiate  heat,  furnish  an  explanation  of  a 
diversity  among  themselves,  in  regard  to  attraction  for  dew,  which 
was  noted  in  the/oregoing  part  of  this  Essay.  Gold,  silver,  copper, 
and  tin,  are  there  said  to  resist  the  formation  of  dew  more  strongly 
than  other  substances  of  the  same  class ;  but  these  metals  according 
to  Mr.  Leslie  radiate  heat  the  most  sparingly.  On  the  other  hand, 
lead,  iron,  and  steel,  which  according  to  the  same  author  radiate  heat 
more  copiously  than  the  former  metals,  were  found  by  me  to  acquire 

*Page  43. 


52  ESSAY  ON  DEW. 

dew  more  readily.  I  do  not  know  if  the  radiating  power  of  platina 
has  been  ascertained  by  direct  experiments ;  but  as  its  conducting 
power  is  small  its  radiation  must  be  great,  since  these  qualities  exist 
always  in  opposite  degrees  in  the  same  substance ;  and  I  have  ac- 
cordingly observed  it  to  be  dewed,  while  the  four  first-mentioned 
metals  were  dry.  I  am  ignorant  both  of  the  radiating  and  the  con- 
ducting power  of  zinc,  as  determined  by  ordinary  experiments  ;  but 
I  infer  from  its  being  more  easily  dewed  than  gold  or  silver,  that  it 
radiates  heat  more  copiously  than  they  do  ;  unless  indeed,  the  pieces 
•which  I  used,  from  having  had  their  surfaces  roughened  by  friction 
with  sand,  which  was  employed  to  brighten  them,  had  acquired  a 
radiating  power  greater  than  that  possessed  by  polished  pieces, 
agreeably  to  the  results  of  some  of  Mr.  Leslie's  experiments.* 

XI.  Thinking  it  probable,  that  black  bodies  might  radiate  more 
heat  to  the  sky  at  night  than  white,  I  placed  upon  grass,  on  five 
different  evenings,  equal  parcels  of  black  and  white  wool.  On  four 
of  the  succeeding  mornings  the  black  wool  was  found  to  have  ac- 
quired a  little  more  dew  than  the  white;  whence  I  inferred  that  it 
had,  in  consequence  of  its  colour,  radiated  a  little  more  heat.  But  I 
afterwards  remarked  that  the  white  wool  was  somewhat  coarser  than 
the  black;  which  circumstance  alone  was  sufficient  to  occasion  a 
difference  in  their  quantities  of  moisture.  Another  night  I  laid  on 
the  raised  board  a  piece  of  pasteboard  covered  with  white  paper, 
and  close  to  this  a  second  piece  similar  to  the  former  in  every  re- 
spect, except  that  it  was  covered  with  paper  blackened  with  ink. 
At  daylight  I  saw  hoarfrost  upon  both  pieces ;  but  the  black  seemed 
to  have  a  greater  quantity  than  the  white.  A  doubt,  however,  after- 
wards arose  upon  the  accuracy  of  this  experiment  likewise  ;  for  as 
the  light  was  faint  when  I  viewed  the  two  surfaces,  the  quantity  of 
hoarfrost,  though  equal  on  both,  might  have  appeared  greater  on  the 
black  than  on  the  white,  from  the  contrast  of  its  colour  with  that  of 
the  former  surface.  But  trials  of  this  kind,  as  Mr.  Lesliet  has  ob- 
served, never  afford  firm  conclusions ;  since  a  black  body  must 
always  differ  from  a  white  in  one  or  more  chemical  properties,  and 

*  I  once  intended  to  subjoin  here  an  explanation  of  some  very  curious  observa- 
tions by  Mr.  Benedict  Prevost  on  dew,  which  were  published,  first  in  the  44th 
volume  of  the  French  Annals  of  Chemistry,  and  afterwards  by  Mr.  Peter  Prevost 
of  Geneva,  in  his  Essay  on  Radiant  Heat ;  but  fearing  to  be  very  tedious,  I  have 
since  given  up  the  design.  I  will  say,  however,  that  if  to  what  is  now  generally 
known  on  the  different  modes,  in  which  heat  is  communicated  from  one  body  to 
another,  be  added  the  two  following  circumstances  ;  that  substances  become 
colder  by  radiation  than  the  air,  before  they  attract  dew ;  and  that  bright  metals 
•when  exposed  to  a  clear  sky  at  night,  become  colder  than  the  air  much  less 
readily  than  other  bodies  ;  the  whole  of  the  appearances  observed  by  Mr.  Prevost 
may  be  easily  accounted  for. 

Note  to  second  edition."]  I  found,  shortly  after  the  publication  of  the  former 
edition  of  this  Essay,  that  the  learned  Dr.  Young  had  several  years  before,  in 
his  great  work  on  Natural  Philosophy,  employed  the  principle  of  the  radiation 
of  heat  to  account  for  several  of  the  facts  observed  by  Mr.  B.  Prevost.  On  the 
subject  of  Dr.  Young's  explanation,  I  have  spoken  somewhat  fully  in  the  28th 
number  of  Dr.  Thomson's  Annals  of  Philosophy. 

t  On  Heat,  p.  95. 


THE  THEORY  OF  DEW.  53 

this  difference  may  of  itself  be  competent  to  produce  a  diversity  in 
their  powers  to  radiate  heat. 

With  the  view  to  render  the  subject  less  complicated,  I  have 
hitherto  treated  of  dew  as  if  it  were  altogether  derived  from  watery 
vapour  previously  diffused  through  the  atmosphere;  this  appearing 
to  me  to  be  by  far  its  most  considerable  source,  and  none  of  my  con- 
clusions of  any  importance  being  liable  to  be  affected,  even  by  the 
establishment  of  a  contrary  opinion.  Other  writers,  however,  have 
regarded  dew  as  being  entirely  the  product  of  vapour  emitted  during 
the  night,  by  the  earth  and  plants  upon  it.  According  to  this 
theory  dew  is  said  to  rise. 

The  first  trace,  which  I  have  found  of  the  opinion  that  dew  rises 
from  the  earth  at  night,  occurs  in  the  History  of  the  Academy  of 
Sciences  for  1697.  It  is  mentioned  there  briefly  and  obscurely, 
and  was,  probably,  shortly  forgotten  ;  for  Gersten,  who  advanced 
it  anew  in  1733,  held  himself  to  be  its  author.  Musschenbroek  and 
Dufay  embraced  it  immediately  after  Gersten  ;  but  the  former  soon 
admitted,  that  dew  sometimes  falls.  As  far  as  I  have  learned,  no 
writer  upon  dew  has  since  ascribed  its  total  production  to  vapour, 
emitted  by  the  earth  at  night,  except  Mr.  Webster  of  New  Eng- 
land.* But  this  opinion  is  frequently  advanced  in  conversation  by 
persons  not  much  accustomed  to  philosophical  pursuits,  chiefly,  I 
think,  because  it  contradicts  a  popular  belief. 

The  only  argument  used  by  the  French  academicians,  in  support 
of  their  opinion,  is,  if  I  understand  it  rightly,  that  as  much  dew  is 
observed  under  an  inverted  glass-bell  as  in  any  other  situation. 
But  admitting,  for  a  moment,  this  to  be  true,  they  would  not  thus 
prove  that  the  ground  is  the  only  source  of  that  fluid. 

Gersten  was  led  to  think  that  dew  rises  from  the  earth,  by  often 
finding  grass,  and  low  shrubs,  moistened-  with  it,  while  trees  were 
dry.  Respecting  this  fact,  I  shall  add  nothing  to  what  I  have  lately 
said  upon  it.  But  his  chief  argument  is  derived  from  another  fact 
related  in  the  first  part  of  this  Essay,  which  is,  that  a  plate  of  metal, 
laid  upon  bare  earth  on  a  dewy  night,  will  remain  dry  on  its  upper 
surface,  while  it  becomes  moist  on  the  lower.  This  also  is  easily 
explicable  by  what  has  already  been  mentioned  by  me.  For  the 
lower  side  of  the  metal,  in  consequence  of  the  upper  being  in  con- 
tact with  the  air  and  being  exposed  to  a  clear  sky,  is  colder  than  the 
earth  a  little  below  the  surface,  and  therefore  condenses  the  vapour, 
which  strikes  against  its  bottom  ;  while  the  upper  side,  from  being 
frequently  warmer,  and  never  more  than  a  little  colder  than  the  air, 
is  for  the  most  part  unable  to  condense  the  watery  vapour  of  the 
atmosphere. t  Gersten,  moreover,  describes  several  appearances 

*  Mem.  of  American  Acad.  vol.  iii. 

f  I  have,  in  like  manner,  observed,  on  a  cloudy  night,  a  piece  of  glass,  laid 
over  an  earthen  pan  containing  water  and  placed  upon  the  ground,  to  be  wet  on 
its  lower  side,  while  the  upper  was  dry  ;  the  glass  being,  in  this  situation,  suffi- 
ciently cold  to  condense  the  vapour  of  water  heated  by  the  earth,  but  not  enough 
so  to  condense  the  watery  vapour  of  the  atmosphere. 

OCT.  1838.-—  U  6 


54  ESSAY  ON  DEW. 

himself,  which  refute  his  opinion.  He  mentions,  for  example,  that 
the  higher  parts  of  shrubs  are  more  dewed  than  the  lower  ;  that 
metallic  plates,  placed  horizontally  in  the  air,  are  as  much  dewed 
on  their  superior  as  on  their  inferior  surfaces  ;  and  that  convex  and 
cylindrical  bodies,  suspended  in  the  air,  the  latter  having  a  position 
parallel  to  the  horizon,  are  dewed  only  on  their  upper  parts. 

The  principal  reason  given  by  Dufay  for  the  rising  of  dew  is, 
that  it  appears  more  early  on  bodies  near  to  the  earth  than  on  those 
which  are  at  a  greater  height.  But  this  fact  readily  admits  of  an 
explanation  on  other  grounds  that  have  already  been  mentioned. 
1.  The  lower  air,  on  a  clear  and  calm  evening,  is  colder  than  the 
upper,  and  will,  therefore,  be  sooner  in  a  condition  to  deposit  a 
part  of  its  moisture.  2.  It  is  less  liable  to  agitation  than  the  upper. 
3.  It  contains  more  moisture  than  the  upper,  from  receiving  the 
last  which  has  risen  from  the  earth,  in  addition  to  what  it  had  pre- 
viously possessed,  in  common  with  other  parts  of  the  atmosphere. 
Dufay  attempted  to  strengthen  his  argument,  by  exposing,  on  three 
dewy  nights,  similar  substances  at  different  heights  from  the  ground, 
expecting  that  the  lower  would  always  acquire  more  moisture  than 
the  upper ;  but,  upon  all  the  nights,  some  one  of  the  lower  substan- 
ces acquired  less  moisture  than  some  one  of  the  higher. 

Mr.  Webster  has  advanced  no  new  fact  in  favour  of  the  opinion 
of  which  I  am  speaking. 

Enough  having  be  said  to  prove,  that  dew  is  not  entirely  the 
product  of  vapour  rising  from  the  earth  at  night,  I  shall  next  show, 
that  it  often  occurs  when  this  cause  can  have  little  or  no  operation. 

1.  It  appears  from  Hasselquist  and  Bruce,  that  in  Egypt,  shortly 
before  the  rising  of  the  Nile,  and  consequently  when  the  ground 
there  is  in  its  dryest  state,  dew  becomes   exceedingly  plentiful, 
though  little  or  none  had  formed  before,  while  the  earth  was  some- 
what less  dry.     The  cause  evidently,  is,  as  was  formerly  mentioned, 
the  moist  air  brought  from  the  Mediterranean  by  the  north  wind, 
which  then  prevails. 

2.  Mr.  Webster,  speaking  of  hoar-frost,  which  he  properly  re- 
gards  as   frozen  dew,   candidly   says,  though    it   overthrows   his 
opinion  :   "  This  frost  appears  when  the  surface  of  the  earth  is 
sealed  with  frost,  and  of  course  the  vapour  of  which  it  is  formed, 
cannot  at  the  time,  perspire  from  the  earth." 

3.  I  have  myself,  at  all  seasons  of  the  year,  frequently  observed 
wool,  upon  the  middle  of  the  raised  board,  and  therefore  out  of  the 
way  of  vapour  rising  from  the  ground,  to  acquire  more  dew   than 
wool  laid  upon  the  grassplat. 

4.  The  bodies  that  condense  the  rising  vapour  must  necessarily 
be  colder  than  it ;  but,  as  they  are  likewise,  according  to  the  opinion 
under  view,  of  the  same  temperature  with  the  air  surrounding  them, 
this  also  should  condense  the  rising  vapour.     Dew,  therefore,  should 
never  appear  in  any  considerable  quantity,  without  being  accompanied 
with  fog  or  mist.     Now  I  can  assert,  after  much  attention  to  this 
point,  that  the  formation  of  the  most  abundant  dew  is  consistent 


THE  THEORY  OF  DEW.  55 

with  a  pellucid  state  of  the  atmosphere.  Hasselquist  makes  a  similar 
observation  with  regard  to  Egypt  ;  where,  during  the  season  re- 
markable for  the  most  profuse  dews,  "the  nights,"  he  says,  "are  as 
resplendent  with  stars,  in  the  midst  of  summer,  as  the  lightest  and 
clearest  winter  nights  in  the  north." 

But,  although  these  facts  prove  that  copious  dews  may   occur 
with  little  or  no  contribution  by  vapour  immediately  rising  from 
the  earth,  it  must  yet  be  admitted,  that  some  of  the  moisture,  which 
forms  during  clear  and  still  weather,  on  bodies  situated  upon  or  near 
its  surface,  is  in  most  cases  to  be  attributed  to  this  source  ;  since,  in 
my  experiments,  substances  on  the  raised  board  became  much  later 
moist  than  others  on  the  ground,  though  equally  cold  with  them. 
The  quantity  from  this  cause,  however,  can  never  be  great.  „.   For 
in  the  first  place,  until  the  air  be  cooled  by  the  substance  attractive 
of  dew,  with  which  it  comes  in  contact,  below  its  point  of  repletion 
with  moisture,  it  will  be  always  in  a  condition  to  take  up  that  which 
has  been  deposited  upon  grass,  or  other  low  bodies,  by  warm  vapour 
emitted  by  the  earth  ;  just  as  the  moisture  formed  upon  a  mirror  by 
our  breath  is,  in  temperate  weather,  almost  immediately  carried 
away  by  the  surrounding  air.     Accordingly  ;  I  have  sometimes,  in 
serene  and  still  weather,  observed  dew  to  appear  sparingly  upon 
grass  in  the  shade,  several  hours  before  sunset,  and  to  continue  in 
nearly  the  same  quantity  till  about  sunset,  when  it  would  increase 
considerably,  at  the  time  that  the  same  fluid  began  to  show  itself  on 
the  raised  board.     In  the  second  place  ;  though  bodies  situated  on 
the  ground,  after  they  have  been  made  sufficiently  cold,  by  radiation, 
to  condense  the  vapour  of  the  atmosphere,  will  be  able  to  retain  the 
moisture  which   they  acquire  by  condensing  the  vapour   of  the 
earth  ;  yet,  before  this  happens,  the  rising  vapour  must  have  been 
greatly  diminished,  by  the  surface  of  the  ground   having  become 
much  colder.     These  considerations,  added  to  the  fact,  that  sub- 
stances on  the  raised  board  attracted  rather  more  dew,  throughout 
the  night,  than  similar  substances  lying  on  the  grass,  warrant  me  to 
conclude,  that  on  nights  favourable  to  the  production  of  dew,  only 
a  very  small  part  of  what  occurs  is  owing  to  vapour  rising  from 
the  earth  ;  though  I  am  acquainted  with  no  means  of  determining 
the  proportion  of  this  part  to  the  whole.     On  the  other  hand,  how- 
ever, in  a  cloudy  night,  all  the  dew  that  appears  upon  grass  may 
sometimes  be  attributed  to  a  condensation  of  the  earth's  vapour  ; 
since  I  have  several  times,  in  such  nights,  remarked  the  raised  board 
to  be  dry,  while  the  grass  was  moist.     These  nights  were  calm,  and 
evaporation  from  the  grass  consequently  not  copious.    When  evapora- 
tion on  cloudy  nights  was  assisted  by  wind,  dew  has  never,  as  was 
mentioned  in  the  first  part  of  this  Essay,  been  any  where  observed 
by  me.* 

*  The  interval  between  the  first  appearance  of  dew  in  the  afternoon  on  grass  in 
shaded  places,  and  sunset,  was  formerly  said  by  me,  on  the  authority,  however, 
of  only  a  few  observations,  to  be  considerably  greater  than  that  between  sunrise 
and  the  ceasing  of  the  formation  of  dew  upon  grass  in  the  morning.  These  ob* 


56  ESSAY  ON  DEW. 

Agreeably  to  another  opinion,  the  dew  found  upon   growing 
vegetables  is  the  condensed  vapour  of  the  very  plants  on  which  it 
appears.     But  this  also  seems  to  me  erroneous  for  several  reasons. 
1.  Dew  forms  as  copiously  upon  dead  as  upon    living  vegetable 
substances.     2.  The  transpired  humour  of  plants  will  be  carried 
away  by  the  air  which  passes  over  them   when  they  are  not  suf- 
ficiently cold  to  condense  the  watery  vapour  contained  in  it ;  unless, 
which  is  almost  never  the  case  if  mist  does  not  already  exist,  the 
general  mass  of  the  atmosphere  be  incapable  of  receiving  moisture 
in  a  pellucid  form.     Accordingly,  on   cloudy  nights,  when  the  air, 
consequently,  can  never  be  cooled  more  than  a   little   below  the 
point  of  repletion  with  moisture,  by  bodies  in  contact  with  it,  dew  is 
never  observed  upon  any  plants  that  are  elevated  a  few  feet  above 
the  ground.     3.  If  a  plant  has  become,  by  radiating  its  heat  to  the 
heavens,  so  cold,  as  to  be  enabled  to  bring  the  air  in  contact  with 
it  below  the  point  of  repletion  with  moisture,  that  which   forms 
upon  it,  from  its  own  transpiration,  will  not  then  indeed  evaporate. 
But  other  moisture  will,  at  the  same  time,  be  communicated  to  it 
by  the  atmosphere  ;  and  when  the  difference  in  the  copiousness  of 
these  two  sources  is  considered,  it  may,  I  think,  be  safely  concluded, 
that  almost  the  whole  of  the  dew  which  will  afterwards  form  on 
the  plant  must  be  derived  from  the  air  ;  more  especially  when  the 
coldness  of  a  clear  night,  and  the  general  inactivity  of  plants  in  the 
absence  of  light,  both  lessening  their  transpiration,  are  taken  into 
account. 

An  experiment  however  has  been  appealed  to  in  proof,  that  the 
dew  of  plants  actually  does  originate  from  fluid  transpired  by  them  ; 
that,  namely,  in  which  a  plant  shut  up  in  an  air-tight  case  becomes 
covered  with  moisture.  But  this  experiment,  if  attentively  exam- 
ined, will  be  found  to  have  little  weight.  First ;  the  inclosed  plant 
being  exempt  from  the  cold  which  its  own  radiation  would  have 
produced  in  its  natural  situation,  on  a  dewy  night,  will  transpire 
a  greater  quantity  of  fluid  than  a  similar  plant  exposed  at  the  same 
time  to  the  open  air.  Again  ;  the  small  quantity  of  air  contained 
in  the  case  must  soon  be  replete  with  moisture,  after  which,  the 
whole  of  what  is  further  emitted  by  the  plant  will  necessarily 
assume  the  form  of  a  fluid,  whatever  may  be  the  condition  of  the 
external  atmosphere  ;  whereas,  during  even  the  clearest  night,  only  a 
part  of  the  smaller  quantity  of  moisture,  emitted  by  the  exposed 
plant,  will  be  condensed  on  its  surface.  In  the  last  place  ;  notwith- 

servations  were  made  on  spots  exposed  during  the  greater  part  of  the  day  to  the 
sun.  In  such  places,  the  heat  acquired,  from  the  sun,  by  the  uppermost  layer 
of  earth,  will  be  longer  retained  than  that  acquired  by  the  grass,  which  will, 
therefore,  be  sufficiently  cool,  soon  after  the  heat  of  the  day  has  declined,  to  con- 
dense a  part  of  the  vapour  then  copiously  rising  from  the  earth  ;  whereas  in  the 
morning,  both  less  vapour  will  rise,  the  surface  of  the  earth  having  now  lost  a 
great  part  of  its  heat,  and  a  less  proportion  of  that  which  does  rise  will  be  con- 
densed by  the  grass  as  the  temperature  of  this  body  now  more  nearly  approaches 
that  of  the  ground,  from  first  receiving  the  heat  of  the  sun  reflected  from  the 
atmosphere  and  other  substances. 


APPEARANCES  CONNECTED  WITH  DEW.  57 

standing  the  circumstances  which  favour  the  appearance  of  moisture 
upon  inclosed  plants  from  their  own  transpiration,  still  the  quantity 
observed  on  them  is  said  to  be,  for  I  have  made  no  experiment 
myself  respecting  this  matter,  much  less  considerable  than  what  is 
seen  upon  plants  of  the  same  kind,  exposed  to  the  air  for  the  same 
time,  during  a  calm  and  serene  night. 


PART  III. 

OF  SEVERAL  APPEARANCES  CONNECTED  WITH  DEW. 

THERE  are  various  occurrences  in  nature  which  seem  to  me 
strictly  allied  to  dew,  though  their  relation  to  it  be  not  always  at 
first  sight  perceivable.  The  statement  and  explanation  of  several 
of  these  will  form  the  concluding  part  of  the  present  Essay. 

I.  I  observed  one  morning  in  winter,  that  the  insidesof  the  panes 
of  glass  in  the  windows  of  my  bedchamber  were  all  of  them  moist, 
but  that  those,  which  had  been  covered  by  an  inside  shutter  during 
the  night,  were  much  more  so  than?others  which  had  been  uncovered. 
Supposing  that  this  diversity  of  appearance  depended  upon  a  differ- 
ence of  temperature,  I  applied  the  naked  bulbs  of  two  delicate  ther- 
mometers to  a  covered  and  uncovered  pane  ;  on  which  I  found,  that 
the  former  was  3°  colder  than  the  latter.  The  air  of  the  chamber, 
though  no  fire  was  kept  in  it,  was  at  this  time  ll£°  warmer  than 
that  without.  Similar  experiments  were  made  on  many  other 
mornings,  the  results  of  which  were  ;  that  when  the  warmth  of  the 
internal  air  exceeded  that  of  the  external,  from  8°  to  18°,  the  tem- 
perature of  the  covered  panes  would  be  from  1°  to  5°  less  than  that  of 
the  uncovered  ;  that  the  covered  were  sometimes  dewed,  while  the 
uncovered  were  dry ;  that  at  other  times  both  were  free  from  mois- 
ture :  that  the  outsides  of  the  covered  and  uncovered  panes  had 
similar  differences  with  respect  to  heat,  though  not  so  great  as  those 
of  the  inner  surfaces  ;  and  that  no  variation  in  the  quantity  of  these 
differences  was  occasioned  by  the  weather's  being  cloudy  or  fair, 
provided  the  heat  of  the  internal  air  exceeded  that  of  the  external 
equally  in  both  of  those  states  of  the  atmosphere. 

The  remote  reason  of  these  differences  did  not  immediately  pre- 
sent itself.  I  soon,  however,  saw,  that  the  closed  shutter  shielded 
the  glass  which  it  covered  from  the  heat  that  was  radiated  to  the 
windows  by  the  walls  and  furniture  of  the  room,  and  thus  kept  it 
nearer  to  the  temperature  of  the  external  air,  than  those  parts  could 
be,  which  from  being  uncovered  received  the  heat  emitted  to  them 
by  the  bodies  just  mentioned. 

In  making  these  experiments  I  seldom  observed  the  inside  of 
any  pane  to  be  more  than  a  little  damped,  though  it  might  be  from 
S°  to  12°  colder  than  the  general  mass  of  the  air  in  the  room  ;  while* 


58  ESSAY  ON  DEW. 

in  the  open  air  I  had  often  found  a  great  dew'to  form  on  substances 
only  3°  or  4°  colder  than  the  atmosphere.  This  at  first  surprised 
me  :  but  the  cause  now  seems  plain.  The  air  of  the  chamber  had 
once  been  a  portion  of  the  external  atmosphere,  and  had  afterwards 
been  heated,  when  it  could  receive  little  accession  to  its  original 
moisture.  It  consequently  required  being  cooled  considerably, 
before  it  was  even  brought  back  to  its  former  nearness  to  repletion 
with  water  ;  whereas  the  whole  external  air  is  commonly,  at  night, 
nearly  replete  with  moisture,  and  therefore  readily  precipitates  dew 
on  bodies  only  a  little  colder  than  itself. 

When  the  air  of  a  room  is  warmer  than  the  external  atmosphere, 
the  effect  of  an  outside  shutter,  on  the  temperature  of  the  glass  of 
the  window,  will  be  directly  opposite  to  what  has  been  just  stated  ; 
since  it  must  prevent  the  radiation  into  the  atmosphere  of  the  heat 
of  the  chamber  transmitted  through  the  glass. 

II.  Count  Rumford*  appears  to  have  rightly  conjectured,  that  the 
inhabitants  of  certain  hot  countries,  who  sleep  at  nights  on  the  tops 
of  their  houses,  are  cold  during  this  exposure,  by  the  radiation  of 
their  heat  to  the  sky  ;  or,  according  to  his  manner  of  expression, 
by  receiving  frigorific  rays  from  the  heavens.  Another  fact  of  this 
kind  seems  to  be  the  greater  chill,  which  we  often  experience  upon 
passing,  at  night,  from  the  cover  of  a  house  into  the  open  air,  than 
might  have  been  expected  from  the  cold  of  the  external  atmosphere. 
The  cause,  indeed,  is  said  to  be  the  quickness  of  transition  from  one 
situation  to  another.  But,  if  this  were  the  whole  reason,  an  equal 
chill  would  be  felt  in  the  day,  when  the  difference,  in  point  of  heat, 
between  the  internal  and  external  air  was  the  same  as  at  night, 
which  is  not  the  case.  Besides  ;  if  I  can  trust  my  own  observation, 
the  feeling  of  cold  from  this  cause  is  more  remarkable  in  a  clear 
than  in  a  cloudy  night,  and  in  the  country  than  in  towns.  The  fol- 
lowing appears  to  be  the  manner  in  which  these  things  are  chiefly 
to  be  explained. 

During  the  day,  our  bodies  while  in  the  open  air,  although  not 
immediately  exposed  to  the  sun's  rays,  are  yet  constantly  deriving 
heat  from  them,  by  means  of  the  reflection  of  the  atmosphere. 
This  heat,  though  it  produces  little  change  on  the  temperature  of  the 
air  which  it  traverses,  affords  us  some  compensation  for  what  we 
radiate  to  the  heavens.  At  night  also,  if  the  sky  be  overcast,  some 
compensation  will  be  made  to  us,  both  in  towns  and  in  the  country, 
though  in  a  less  degree  than  during  the  day,  as  the  clouds  will  remit 
towards  the  earth  no  inconsiderable  quantity  of  heat.  But  on  a  clear 
night,  in  an  open  part  of  the  country,  nothing  almost  can  be  returned 
to  us  from  above,  in  place  of  the  heat  which  we  radiate  upwards. 
In  towns,  however,  some  compensation  will  be  afforded,  even  on 
the  clearest  nights,  for  the  heat  which  we  lose  in  the  open  air,  by 
that  which  is  radiated  to  us  by  the  surrounding  buildings. 

To  our  loss  of  heat  by  radiation  at  times  that  we  derive  little 

*  Phil.  Trans.  1804.  p.  182. 


APPEARANCES  CONNECTED  WITH  DEW.  59 

compensation  from  the  radiation  of  other  bodies,  is  probably  to  be 
attributed  a  great  part  of  the  hurtful  effects  of  the  night  air.  Des- 
cartes* says  that  these  are  not  owing  to  dew,  as  was  the  common 
opinion  of  his  contemporaries,  but  to  the  descent  of  Certain  noxious 
vapours,  which  having  been  exhaled  from  the  earth  during  the  heat 
of  the  day,  are  afterwards  condensed  by  the  cold  of  a  serene  night. 
The  effects  in  question  certainly  cannot  be  occasioned  by  dew,  since 
that  fluid  does  not  form  upon  a  healthy  human  body,  in  temperate 
climates  ;  but  they  may,  notwithstanding,  arise  from  the  same 
cause  that  produces  dew  on  those  substances,  which  do  not,  like 
the  human  body,  possess  the  power  of  generating  heat  for  the  supply 
of  what  they  lose  by  radiation  or  any  other  means. 

III.  I  had  often,  in  the  pride  of  half  knowledge,  smiled  at  the 
means  frequently  employed  by  gardeners,  to  protect  tender  plants 
from  cold,  as  it  appeared  to  me  impossible  that  a  thin  mat.  or  any 
such  flimsy  substance,  could  prevent  them  from  attaining  the  tem- 
perature of  the  atmosphere,  by  which  alone  I  thought  them  liable 
to  be  injured.  But,  when  I  had  learned  that  bodies  on  the  surface 
of  the  earth  become,  during  a  still  and  serene  night,  colder  than  the 
atmosphere,  by  radiating  their  heat  to  the  heavens,  I  perceived  im- 
mediately a  just  reason  for  the  practice  which  I  had  before  deemed 
useless.  Being  desirous,  however,  of  acquiring  some  precise  infor- 
mation on  this  subject,  I  fixed  perpendicularly  in  the  earth  of  a  grass- 
plat,  4  small  sticks,  and  over  their  upper  extremities,  which  were  6 
inches  above  the  grass,  and  formed  the  corners  of  a  square,  the  sides 
of  which  were  2  feet  long,  drew  tightly  a  very  thin  cambric  hand- 
kerchief. In  this  disposition  of  things,  therefore,  nothing  existed 
to  prevent  the  free  passage  of  air  from  the  exposed  grass,  to  that 
which  was  sheltered,  except  the  4  small  sticks,  and  there  was  no 
substance  to  radiate  heat  downwards  to  the  latter  grass,  except  the 
cambric  handkerchief.  The  temperature  of  the  grass,  which  was  thus 
shielded  from  the  sky,  was  upon  many  nights  afterwards  examined 
by  me,  and  was  always  found  higher  than  that  of  neighbouring 
grass  which  was  uncovered,  if  this  was  colder  than  the  air.  When 
the  difference  in  temperature,  between  the  air  several  feet  above  the 
ground  and  the  unsheltered  grass,  did  not  exceed  5°,  the  sheltered 
grass  was  about  as  warm  as  the  air.  If  that  difference,  however, 
exceeded  5°,  the  air  was  found  to  be  somewhat  warmer  than  the 
sheltered  grass.  Thus,  upon  one  night,  when  fully  exposed  grass 
was  11°  colder  than  the  air,  the  latter  was  3°  warmer  than  the  shel- 
tered grass ;  and  the  same  difference  existed  on  another  night,  when 
the  air  was  14°  warmer  than  the  exposed  grass.  One  reason  for  this 
difference,  no  doubt,  was  that  the  air,  which  passed  from  the  exposed 
grass,  by  which  it  had  been  very  much  cooled,  to  that  under  the 
handkerchief,  had  deprived  the  latter  of  part  of  its  heat ;  another, 
that  the  handkerchief,  from  being  made  colder  than  the  atmosphere 
by  the  radiation  of  its  upper  surface  to  the  heavens,  would  remit 

Meteorolog.  c.  vi. 


60  ESSAY  ON  DEW. 

somewhat  less  heat  to  the  grass  beneath  than  what  it  received 
from  that  substance.  But  still,  as  the  sheltered  grass,  notwithstand- 
ing these  drawbacks,  was  upon  one  night,  as  may  be  collected  from 
the  preceding  relation,  8°,  and  upon  another  11°,  warmer  than  grass 
fully  exposed  to  the  sky,  a  sufficient  reason  was  now  obtained  for 
the  utility  of  a  very  slight  shelter  to  plants,  in  averting  or  les- 
sening injury  from  cold,  on  a  still  and  serene  night. 

In  the  next  place  ;  in  order  to  learn  whether  any  difference  would 
arise  from  placing  the  sheltering  substance  at  a  much  greater  distance 
from  the  ground,  I  had  4  slender  posts  driven  perpendicularly  into 
the  soil  of  a  grass  field,  and  had  them  so  disposed  in  other  respects 
that  their  upper  ends  were  6  feet  above  the  surface,  and  formed  the 
angular  points  of  a  square  having  sides  8  feet  in  length.  Lastly  ; 
over  the  tops  of  the  posts  was  thrown  an  old  ship  flag  of  a  very 
loose  texture.  Concerning  the  experiments  made  by  means  of 
this  arrangement  of  things,  I  shall  only  say,  that  they  led  to  the 
conclusion,  as  far  as  the  events  of  different  nights  could  rightly  be 
compared,  that  the  higher  shelter  had  the  same  efficacy  with  the 
lovvera  in  preventing  the  occurrence  of  a  cold  upon  the  ground,  in 
a  clear  night,  greater  than  that  of  the  atmosphere,  provided  the 
oblique  aspect  of  the  sky  was  equally  excluded  from  the  spots  on 
which  my  thermometers  were  laid. 

On  the  other  hand  ;  a  difference  in  temperature,  of  some  magni- 
tude, was  always  observed  on  still  and  serene  nights,  between 
bodies  sheltered  from  the  sky  by  substances  touching  them,  and 
similar  bodies,  which  were  sheltered  by  a  substance  a  little  above 
them.  I  found,  for  example,  upon  one  night,  that  the  warmth  of 
grass,  sheltered  by  a  cambric  handkerchief  raised  a  few  inches  in 
the  air,  was  3°  greater  than  that  of  a  neighbouring  piece  of  grass, 
which  was  sheltered  by  a  similar  handkerchief  actually  in  contact 
with  it.  On  another  night,  the  difference  between  the  temperatures 
of  two  portions  of  grass,  shielded  in  the  same  manner  as  the  two 
above-mentioned  from  the  influence  of  the  sky,  was  4°.  Possibly, 
experience  has  long  ago  taught  gardeners  the  superior  advantage  of 
defending  tender  vegetables,  from  the  cold  of  clear  and  calm  nights, 
by  means  of  substances  not  directly  touching  them ;  though  I  do  not 
recollect  ever  having  seen  any  contrivance  for  keeping  mats,  or 
such  like  bodies,  at  a  distance  from  the  plants  which  they  were 
meant  to  protect. 

Walls,  I  believe,  as  far  as  warmth  is  concerned,  are  regarded  as 
useful  during  a  cold  night,  to  the  plants  which  touch  them,  or  are 
near  to  them,  only  in  two  ways ;  first,  by  the  mechanical  shelter 
wh'ich  they  afford  against  cold  winds,  and  secondly,  by  giving  out 
the  heat  which  they  had  acquired  during  the  day.  It  appearing  to 
me,  however,  that,  on  clear  and  calm  nights,  those  on  which  plants 
frequently  receive  much  injury  from  cold,  walls  must  be  beneficial 
in  a  third  way,  namely,  by  preventing,  in  part,  the  loss  of  heat, 
which  they  would  sustain  from  radiation,  if  they  were  fully  exposed 


APPEARANCES  CONNECTED  WITH  DEW.  61 

to  the  sky,  the  following  experiment  was  made  for  the  purpose  of 
determining  the  justness  of  this  opinion. 

A  cambric  handkerchief  having  been  placed,  by  means  of  two 
upright  sticks,  perpendicularly  to  a  grassplat,  and  at  right  angles  to 
the  course  of  the  air,  a  thermometer  was  laid  upon  the  grass  close  to 
the  lower  edge  of  the  handkerchief,  on  its  windward  side.  The 
thermometer  thus  situated  was  several  nights  compared  with 
another  lying  on  the  same  grassplat,  but  on  a  part  of  it  fully  ex- 
posed to  the  sky.  On  two  of  these  nights,  the  air  being  clear  and 
calm,  the  grass  close  to  the  handkerchief  was  found  to  be  4°  warmer 
than  the  fully  exposed  grass.  On  a  third,  the  difference  was  6°. 
An  analogous  fact  is  mentioned  by  Gersten,  who  says,  that  a  hori- 
zontal surface  is  more  abundantly  dewed  than  one  which  is  per- 
pendicular to  the  ground. 

IV.  The  covering  of  snow,  which  countries  in  high  latitudes 
enjoy  during  the  winter,  has  been  very  commonly  thought  to  be 
beneficial  to  vegetable  substances  on  the  surface  of  the  earth,  as  far 
as  their  temperature  is  concerned,  solely  by  protecting  them  from 
the  cold  of  the  atmosphere.  But  were  this  supposition  just,  the 
advantage  of  the  covering  would  be  greatly  circumscribed ;  since 
the  upper  parts  of  trees  and  of  tall  shrubs  are  still  exposed  to  the 
influence  of  the  air.  Another  reason,  however,  is  furnished  for  its 
usefulness,  by  what  has  been  said  in  this  Essay  ;  which  is,  that  it 
prevents  the  occurrence  of  the  cold  which  bodies  on  the  earth  ac- 
quire, in  addition  to  that  of  the  atmosphere,  by  the  radiation  of  their 
heat  to  the  heavens  during  still  and  clear  nights.  The  cause,  indeed, 
of  this  additional  cold,  does  not  constantly  operate  :  but  its  presence, 
during  only  a  few  hours,  might  effectually  destroy  plants  which  now 
pass  unhurt  through  the  winter.  Again  ;  as  things  are,  while  low 
vegetable  productions  are  prevented,  by  their  covering  of  snow, 
from  becoming  colder  than  the  atmosphere  in  consequence  of  their 
own  radiation,  the  parts  of  trees  and  tall  shrubs,  which  rise  above 
the  snow,  are  little  affected  by  cold  from  this  cause.  For  their 
outermost  twigs,  now  that  they  are  destitute  of  leaves,  are  much 
smaller  than  the  thermometers  suspended  by  me  in  the  air,  which 
in  this  situation  very  seldom  became  more  than  2°  colder  than  the 
atmosphere.  The  larger  branches  too,  which,  if  fully  exposed  to 
the  sky,  would  become  colder  than  the  extreme  parts,  are,  in  a 
great  degree,  sheltered  by  them  ;  and,  in  the  last  place,  the  trunks 
are  sheltered  both  by  the  smaller  and  the  larger  parts,  not  to  mention 
that  the  trunks  must  derive  heat,  by  conduction  through  the  roots, 
from  the  earth  kept  warm  by  the  snow.* 

In  a  similar  way  is  partly  to  be  explained  the  manner  in  which 


*  It  may  be  remarked  here,  however,  that  a  thick  covering  of  snow,  while 
it  renders  the  surface  of  the  earth  warmer  than  it  would  otherwise  be,  must 
occasion  the  lower  atmosphere  to  be  colder,  by  preventing  the  passage  of  the 
heat  of  the  ground  to  the  air,  either  by  radiation  or  conduction. 


62  ESSAY  ON  DEW. 


a  layer  of  earth  or  straw  preserves  vegetable  matters  in  our  own 
fields  from  the  injurious  effects  of  cold  in  winter. 

V.  The  bare  mention  of  the  subject  of  this  article  will  be  apt  to 
excite  ridicule,  it  being  an  attempt  to  show  in  what  way  the  expo- 
sure of  animal  substances  to  the  moon's  light  promotes  their  putre- 
faction. I  have  no  certain  knowledge  that  such  an  opinion  pre- 
vails any  where,  at  present,  except  in  the  West  Indies  ;  but  I  con- 
clude, from  various  circumstances,  that  it  exists  also  in  Africa,  and 
that  it  was  carried  thence  by  negro  slaves  to  America.  It  was  en- 
tertained, however,  by  persons  of  considerable  rank  and  intelligence 
among  the  ancients  ;  for  Pliny*  affirms  it  to  be  true,  and  Plutarch, 
after  making  it  a  subject  of  discussion  in  one  of  his  Symposia,t 
admits  it  to  be  well  founded. 

As  moonbeams  communicate  no  sensible  heat  to  the  bodies  on 
which  they  fall,  it  seems  impossible  that  they  can  directly  promote 
putrefaction.  But  still  a  reason  for  ascribing  such  a  power  to  them, 
may  be  derived  from  their  being  received  by  animal  substances,  at 
the  very  time  that  a  real,  but  generally  unnoticed  cause  of  putrefac- 
tion in  warm  climates,  (and  it  is  in  these  alone  the  opinion  I  am 
treating  of  has  ever  prevailed)  is  taking  place,  which  ceases  to  act 
as  soon  as  the  moon's  light  is  excluded. 

The  nights  on  which  a  steady  moonshine  occurs  must  necessarily 
be  clear;  and  nights  which  are  clear  are  almost  always  calm.J  A 
moonshiny  night,  therefore,  is  one  on  which  dew  forms  plentifully ; 
hence  the  expressions  "  roscida"  and  "  rorifera  luna"  employed  by 
Virgil  and  Statius ;  and  hence  also  an  opinion  held,  as  appears  from 
Plutarch,  even  by  philosophers  among  the  ancients,  that  the  moon 
communicates  moisture  to  the  bodies  which  are  exposed  to  its  light.§ 

Animal  substances  are  among  those  which  acquire  dew  in  the 
greatest  quantity.  To  do  this,  indeed,  they  must  previously  become 
colder  than  the  atmosphere;  but  having  acquired  the  moisture  of 
dew  in  addition  to  their  own,  they  will  on  the  following  day  be  in 
that  condition  which  is  known  by  experience  to  favour  putrefac- 
tion most  powerfully  in  hot  climates. 

The  immediate  cause  assigned  here  for  the  quick  putrefaction  of 
animal  substances,  which  have  been  exposed  to  the  moon's  rays  in  a 
hot  country,  is  the  same  as  that  given  by  Pliny  and  Plutarch ;  but 
they  attributed  the  origin  of  this  immediate  cause,  the  additional 
moisture,  to  the  peculiar  humefying  quality  which  they  supposed 
that  luminary  to  possess.  This  false  theory  has  probably  contri- 

*  Lib.  ii.  §  civ.  t  ^ib.  iii.  Prob.  x. 

$  Mr.  De  Luc  has  remarked,  that  clouds  frequently  disappear  soon  after  sunset. 
Idees  sur  la  Meteorologie,  ii.  98.  I  have  often  observed  this  myself,  and  at;the 
same  time  another  fact  of  which  he  takes  no  notice;  namely,  that  the  atmosphere 
is  then  calmer  than  it  had  been  before  sunset.  This  calmness  of  the  air  very 
commonly,  if  not  always,  precedes  the  dissipation  of  the  clouds. 

§  A  kin  to  this  opinion  of  the  ancients  respecting  the  humefying  quality  of  the 
moon,  is  one,  which  has  been  held  by  modern  writers  as  well  as  ancients,  upon 
that  planet's  being  a  cause  of  cold  to  the  bodies  which  receive  its  rays  ;  though 
I  know  of  no  author  who  has  taken  notice  of  this  affinity. 


APPEARANCES  CONNECTED  WITH  DEW.  63 

buted  to  discredit,  with  the  moderns,  the  circumstance  which  it  was 
employed  to  explain.  *» 

VI.  The  last  fact  of  which  I  shall  treat  in  this  Essay,  is  the  forma- 
tion of  ice,  during  the  night  in  Bengal,  while  the  temperature  of  the 
air  is  above  32°. 

I  have  seen  only  two  original  descriptions  of  this  process,  both  of 
which  are  contained  in  the  Philosophical  Transactions ;  the  first  by 
Sir  Robert  Barker,  in  the  65th  volume;  the  other  in  the  83d,  by 
Mr.  Williams. 

According  to  the  method  followed  by  Sir  R.  Barker's  ice-maker, 
square  excavations  2  feet  deep,  and  30  wide,  having  been  formed  in 
a  large  open  plain,  their  bottoms  are  covered  with  sugar-cane,  or 
stems  of  Indian  corn,  dried  to  the  thickness  of  8  inches  or  1  foot. 
On  this  layer,  are  afterwards  placed  in  rows,  near  to  each  other, 
small  unglazed  earthen  pans,  \  of  an  inch  thick,  and  1  inch  and  £ 
deep,  filled  with  boiled  soft  water.  The  pans  are  sufficiently  porous 
to  allow  their  outer  surface  to  appear  moist,  after  water  has  been 
poured  into  them.  Sir.  R.  Barker  adds  ;  that  the  nights  the  most  fa- 
vourable for  the  production  of  ice,  are  those  which  are  the  calmest 
and  most  serene,  and  on  which  very  little  dew  appears  after  mid- 
night; that  clouds  and  frequent  changes  of  wind  are  certain  pre- 
ventives of  its  formation  ;  and  that,  although  ice  is  thus  very  readily 
procured  by  art  in  Bengal,  during  the  winter,  it  scarcely  ever  occurs 
there  naturally. 

The  process  described  by  Mr.  Williams  must,  from  its  extent, 
300  persons  being  employed  in  it,  have  been  carried  on  for  profit, 
and  would,  consequently  be  conducted  in  the  most  economical  man- 
ner. A  piece  of  ground  nearly  level,  containing  about  4  acres,  was 
divided  into  square  plats,  from  4  to  5  feet  wide,  which  were  sur- 
rounded by  little  mounds  of  earth  4  inches  high.  In  these  inclosures, 
previously  filled  with  dry  straw,  or  sugar-cane  haum,  were  placed 
as  many  broad,  shallow,  unglazed  earthen  pans,  containing  unboiled 
pump  water,  as  they  could  hold.  The  air  was  generally  very  still 
when  much  ice  was  formed ;  wind  prevented  its  formation  alto- 
gether. In  the  morning,  between  5  and  6h.,  at  which  time  alone 
Mr.  Williams  made  his  observations,  a  thermometer,  with  its  bulb 
naked,  placed  on  the  straw,  amidst  the  freezing  vessels,  was  never 
found  by  him  lower  than  35°;  and  he  has  observed  ice,  when  a 
thermometer  so  placed  was  42°.  Another  thermometer,  suspended 
5^  feet  above  the  ground,  was  commonly  4°  higher  than  that  among 
the  pans.  It  is  possible,  therefore,  that  Mr.  Williams  may  have 
seen  ice  a  little  before  sunrise,  when  the  temperature  of  the  air  was 
46°.  But  granting  this  were  the  fact,  it  would  not  hence  follow, 
that  the  ice  was  formed  while  the  air  possessed  that  heat.  For 
although  the  air  is  generally  held  to  be  in  all  countries  colder  about 
sunrise  than  at  any  other  time,  I  know  from  my  own  observations, 
that  this  is  not  always  the  case  in  England ;  and  similar  exceptions 
may  dccur  in  Bengal.  Sir  H.  Davy  has  said,  in  his  Elements  of 
Chemistry,  that  ice  will  form  in  Bengal  when  the  temperature  of 


64  ESSAY  ON  DEW. 

the  air  is  not  below  50° ;  but  he  has  given  no  authority  for  this 
assertion. 

The  formation  of  ice,  in  the  circumstances  which  have  been 
just  mentioned,  was  attributed  by  Sir  R.  Barker  altogether,  and  by 
Mr.  Williams  in  great  measure,  to  cold  produced  by  evaporation. 
Sir  R.  Barker's  opinion  has  since  been  adopted  by  some  of  our 
most  distinguished  writers  on  Natural  Philosophy,  as  Watson, 
Thompson,  Young,  Davy,  and  Leslie,  apparently,  however,  without 
their  having  fully  considered  it,  as  I  shall  now  attempt  to  show. 

1.  It  is  necessary  for  the  complete  success  of  the  process,  that 
the  air  should  be  very  still  ;  wind,  which  so  greatly    promotes 
evaporation,  prevents  the  freezing  altogether.     Sir  R.  Barker  admits, 
that  the  excavations  in  the  earth  are  made  to  increase  the  stillness 
of  the  air  in  contact  with  the  water  in  the  pans ;  but,  with  the  view  to 
explain  the  utility  of  this  stillness  he  supposes,  in  opposition  to  all 
experience,  that  water  kept  very  quiet  freezes  more  readily,  when 
other  circumstances  are  the  same,  than  if  it  were  a  little  agitated. 

2.  No  proof  is  given,  that  evaporation  from  the  pans   actually 
does  occur  at  the  times  which  are  the  most  favourable  for  the  ap- 
pearance of  ice.     At  any  rate  it  cannot  be  considerable  ;    since 
agreeably  to  what  is  mentioned   by  Sir  R.  Barker,  dew  forms  in  a 
greater  or  less  degree  during  the  whole  of  the  nights  the  most 
productive  of  ice;  and  it  is  not  to  bethought,  as  was  said  upon  a  for- 
mer occasion,  that  one  portion  of  air  will  be  depositing  moisture, 
from  possessing  a  superabundance  of  it,  while  another  in  the  imme- 
diate vicinity  is  receiving  moisture  in  great  quantity,  in  the  state  of 
pellucid  vapour  ;  as  the  latter  fact  can  exist  only  when  the  air  is 
far  removed  from  a  state  of  repletion  with  water. 

3.  If   evaporation  produced   the   cold  under  consideration,  the 
wetting  of  the  straw  or  other  matter,  upon  which  the  pans  are 
placed  would  tend  to  increase  it;  and,  accordingly,  Sir  H.Davy 
affirms  this  to  be  the  case.     But  Mr.  Williams,  who  must  here  be 
regarded  as  the  better  authority  says,  that  it  is  necessary  to  the 
success  of  the  process  that  the  straw  be  dry ;  in  proof  of  which  he 
mentions,  that  when   the  straw  becomes  wet  by  accident,  it  is  re- 
placed ;  and   that  when    he  purposely  wetted  it.   in  some   of  the 
inclosures,  the  formation  of  ice  there  was  always  prevented.     The 
reasons  are  clear.     The  water,  by  softening  the  straw,  renders  it 
easily  compressible  by  the  weight  of  the  pans,  and  at  the  same  time 
fills  up  what  would  otherwise  be  vacant  spaces  among  its  parts. 
The  straw,  therefore,  in  this  condensed  state,  must  afford   a  ready 
passage  to  heat  from  the  earth  to  the  pans,  the  hindrance  of  which 
is  allowed  by  every  person  to  be  the  use  of  it  in  this  process  when 
dry.     Again  ;  the  moisture  which  passes  through  the  straw  to  the 
earth  it  covers,  will  rise  afterwards  in  the  form  of  vapour,  having 
the  same  temperature  with  the  warm  ground,  and  will  communicate 
heat  to  the  pans.     In  the  last  place;  a  part  of  this  vapour  will  be 
condensed   into  water  by  the  pans,  in  consequence  of  which  heat 
must  be  extricated. 


APPEARANCES  CONNECTED  WITH  DEW.  65 

4.  It  is  mentioned  both  by  Sir.  R.  Barker  and  Mr.  Wjlliams,  in, 
support  of  their  opinions,  that  the  pans,  when  new,  are  so  porous, 
that  they  readily  permit  water  to  transude  them  ;  and  that  old  pans, 
which  permit  this  in  a  less  degree  are  less  fit  for  the  making  of  ice. 
But  the  argument,  which  is  hence  derived  by  them,  is  completely 
refuted  by  a  fact  related  by  Mr.  Williams  himself;  for  he  says  that 
the  pans  are  greased  before  they  are  used,  to  prevent  the  adhesion 
of  the  ice  to  their  sides;  since,  if  this  purpose  be  answered,  the 
water  can  never  be  in  contact  with  the  pans,  and  therefore  can 
never  pass  through  them. 

The  real  reason  of  the  less  fitness  of  old  pans  for  the  making  of 
ice  is  perhaps  the  following.  The  production  of  the  cold,  which 
occurs  in  this  process  must  take  place  in  the  water;  since  neither 
the  straw  upon  which  the  pans  are  placed,  nor  the  air  above  them, 
was  ever  found  by  Mr.  Williams  of  so  low  a  temperature  as  32.° 
Whatever,  therefore,  obstructs  the  passage  of  heat  from  the  straw 
to  the  water,  must  favour  the  freezing  of  the  latter.  But  this  will 
be  less  effectually  done  by  an  old  than  by  a  new  pan,  as  the  density 
of  the  former  is  greater,  from  the  grease  forced  into  it  by  rubbing, 
and  from  the  slime  and  sand  that  will  enter  with  the  water  into  its 
pores,  when  these  are  not  entirely  closed  by  the  grease  :  which 
must  often  happen  as  the  smearing  is  performed  only  once  in  three 
our  four  days.  The  difference,  however,  in  effect  betwixt  old  and 
new  pans  must  be  very  small  ;  as  it  does  not  appear  that  the  old  are 
ever  laid  aside  on  account  of  their  unfitness. 

In  a  like  way  may  be  explained,  without  the  aid  of  cold  pro- 
duced by  the  evaporation  of  moisture  from  the  outsides  of  the  pans, 
another  fact  mentioned  by  Mr.  Williams,  that  ice  was  often  found 
by  him  in  those  vessels,  while  water  contained  in  a  china  plate,  sur- 
rounded by  them  had  none  ;  since  the  thin  and  dense  substance  of 
the  plate  must  have  transmitted  more  readily,  than  the  thick  and 
rare  substance  of  the  pans,  the  heat  of  the  straw  to  the  water. 

5.  In  accounting  for  the  making  of  ice  in  Bengal,  it  is  requisite 
to  show,  not  only  how  the  first  film  is  produced,  but  also  in  what 
way  the  thickness  of  this  film  is  afterwards  increased.  If  evapora- 
tion be  the  cause  of  this  increase,  it  follows  that  a  plate  of  ice  in  the 
night-time,  and  in  the  stillest  air,  both  unfavourable  to  that  process, 
must  yet  emit  as  much  moisture,  as  is  necessary  for  the  production 
of  a  cold,  according  to  Mr.  Williams,  of  at  least  14°,  and  according 
to  Sir  H.  Davy  of  at  least  18°  ;  a  conclusion,  as  it  appears  to  me,  of 
itself  sufficient  to  destroy  the  credit  of  the  theory  from  which  it  is 
drawn. 

While  attending  to  this  subject,  I  became  desirous  of  acquiring 
some  knowledge  of  the  degree  of  cold,  which  might  be  produced 
by  evaporation  from  water  contained  in  a  shallow  vessel.  With 
this  view  I  placed  on  a  feather  bed,  situated  between  the  door  and 
window  of  a  room  in  my  house  in  London,  two  china  plates,  into 
one  of  which  as  much  water  was  poured  as  covered  its  bottom  to 
the  depth  of  $  of  an  inch.  The  other  plate  was  kept  dry.  The 
OCT.  1838.— X  7 


66  ESSAY  ON  DEW. 

bulb  of  a  small  thermometer  being  then  applied  to  the  inside  of  the 
bottom  of  each  plate,  I  observed  upon  many  days  in  various  seasons 
of  the  year,  the  difference  between  these  instruments  while  the 
door  and  window  were  open.  I  found  in  consequence,  that  when 
the  temperature  of  the  air  in  the  room  was  75°,  the  highest  at  which 
any  experiment  was  made,  the  thermometer  in  the  plate,  containing 
water,  was  between  6  and  7  degrees  lower  than  the  one  in  the  dry 
plate  ;  that  the  difference  between  these  thermometers  diminished 
gradually  as  the  air  became  colder  ;  and  that  when  the  temperature 
of  the  air  was  40°,  the  lowest  for  which  I  have  any  observation,  the 
difference  was  only  l^0.  At  32°,  therefore  it  would  have  been  very 
small,  and  at  a  few  degrees  below  32  it  would  probably  have  van- 
ished. This  supposition  agrees  with  an  observation  made  by  Mr. 
Wilson  of  Glasgow,  who  found'that  no  cold  was  produced  by  evapo- 
ration from  snow  possessing  a  temperature  of  27°,  though  the  air  in 
the  immediate  neighbourhood  was  purposely  much  agitated  by  him. 

The  conclusions  here  given  by  me,  respecting  the  cold  produced 
by  the  evaporation  of  water,  were  drawn  from  experiments  made 
in  the  day,  while  the  sky  was  clear,  the  air  very  calm,  and  the 
temperature  of  the  atmosphere  stationary.  At  night  and  during  a 
cloudy  day,  the  differences  were  less.  On  the  other  hand,  if  there 
was  any  perceptible  motion  in  the  air,  they  were  greater.  They 
were  also  greater  if  the  heat  of  the  atmosphere  was  increasing  ; 
but  less  if  this^was  decreasing. 

Having  thus,  I  think,  placed  beyond  doubt,  that  the  formation  of 
ice  in  Bengal  is  not  occasioned  by  evaporation,  I  shall  now  state 
several  reasons,  which  have  induced  me  to  believe,  that  it  depends 
upon  the  radiation  of  heat  to  the  heavens. 

1.  This  cause  not  only  exists,  but  exists  in  a  degree  sufficient  for 
the  production   of  the   effect  which  I  attribute  to  it.     For  Mr. 
Wilson  found  the  surface  of  snow,  during  a  clear  and  calm  night,  to  be 
16°  colder  than  air  2  feet  above  it,  the  temperature  of  the  latter  being 
taken  by  a  naked  thermometer  ;  whereas  the  greatest  heat  of  the 
atmosphere  ever  observed   by  Mr.  Williams,  at  the  distance  of  5i 
feet  from  the  ground,  during  the  time  that  he  supposed   ice  to  be 
forming,  was  only  14°  higher  than  the  freezing  point  of  water.     I 
need  say  nothing  of  the  difference  of  18°  related  by  Sir  H.  Davy, 
as  he  does  not  speak  from  his  own  observation,  and  as  he  gives  no 
authority  for  what  he  advances  ;  though  even  this  difference  is  con- 
siderably less  than  what  I  have  attempted  to  show  must  sometimes 
occur  from  the  radiation  of  heat  at  night,  between  the  temperature 
of  air  a  few  feet  above  the  earth  and  that  of  bodies  placed  on  its 
surface. 

It  is  to  be  mentioned  here  also,  that,  according  to  Mr.  Leslie,* 
the  power  of  water  to  radiate  heat  exceeds,  perhaps,  that  of  all  other 
substances. 

2.  Ice  is  chiefly  formed  in  Bengal  during  the  clearest  and  calmest 

*  On  Heat,  p.  80. 


APPEARANCES  CONNECTED  WITH  DEW.  67 

nights  :  and  it  is  on  such  nights  that  the  greatest  cold,  from  radia- 
tion, is  observed  on  the  surface  of  the  earth.  In  Sir  R.  Barker's 
more  refined  mode  of  conducting  the  process,  an  unusual  stillness 
of  the  air,  in  contact  with  the  water  to  be  frozen  is  procured,  by 
placing  the  pans  containing  it  a  little  below  the  level  of  the  ground, 
in  which  situation,  it  was  formerly  shown,  bodies  must  grow  colder 
from  radiation  to  the  heavens  at  night  than  in  any  other. 

3.  The  cold,  by  means  of  which  ice  is  produced  in  Bengal,  ap- 
pears, as  I  think  may  be  inferred  from  what  is  said  by  Sir  R.  Barker, 
in  its  greatest  degree,  like  cold  from  radiation  in  other  substances, 
on  those  still  and  serene  nights,  during  which  little  dew  is  deposited 
by  the  atmosphere. 

4.  Clouds  and  wind  prevent  the  formation  of  ice  in  Bengal  ;  and 
the  same  states  of  the  atmosphere  either  prevent,  or  considerably 
diminish  the  occurrence  of  cold  from  the  radiation  of  heat  at  night 
by  bodies  on  the  ground. 

I  shall  close  this  subject,  by  giving  some  account  of  a  few  attempts 
to  procure  the  freezing  of  water  at  night  in  this  country,  by  ex-' 
posing  it  to  air  of  a  temperature  higher  than  that  of  32°.  These 
were  made  by  me  in  1812,  at  my  usual  place  of  experiment,  which 
was  formerly  stated  to  be  not  well  adapted  for  the  appearance  of  a 
great  cold  from  radiation,  and  on  nights  not  among  the  most  favour- 
able to  such  an  undertaking,  even  of  those  which  occur  in  this 
country.  It  is  proper  also  to  mention,  that  I  was  then  less  able  to 
conduct  such  experiments,  and  to  make  use  of  them,  than  I  after- 
wards became  from  a  longer  attention  to  similar  objects. 


EXPERIMENT    I. 

With  a  view  to  imitate  the  method  of  making  ice  described  by 
Sir  R.  Barker,  I  had  a  pit  dug,  on  the  evening  of  the  3d  of  May, 
in  the  middle  of  the  garden  so  often  spoken  of,  4i  feet  long,  3  wide, 
and  2  deep.  It  consequently  had  the  same  depth  as  the  excavations 
mentioned  by  that  gentleman,  but  was  considerably  less  in  its  other 
dimensions.  Clean  dry  straw  was  then  strewed,  to  the  height  of  a 
foot,  over  the  bottom  of  the  pit.  On  the  straw  were  next  laid  a 
number  of  small  shallow  earthen  pans,  a  part  of  which  were  glazed, 
and  a  part  unglazed.  In  the  last  place ;  all  the  pans  were  filled  with 
soft  water,  which  had  been  boiled  on  the  same  evening.  Contrary  to 
my  expectation,  the  unglazed  pans  remained  as  dry  on  the  outside, 
after  water  had  been  poured  into  them,  as  those  which  were  glazed. 
I  conclude,  therefore,  that  the  former  were  more  dense  in  their 
substance,  than  the  unglazed  pans  used  in  India  ;  and  that  their 
density  was  probably  the  reason,  why  ice  did  not  afterwards  form 
in  them  sooner  than  in  the  glazed  pans  which  were  employed  by 
me. 

Two  pans,  containing  boiled  water,  were  set  upon  the  grassplat, 


68  ESSAY  ON  DEW. 

at  a  little  distance  from  the  pit.  A  watch-glass  filled  with  boiled 
water  was  also  placed  upon  the  grassplat,  and  another  was  laid  upon 
the  raised  board,  which  had  been  thinly  covered  with  sand.  All 
these  arrangements  were  not  completed  before  lOh.  at  night. 

At  Ih.  in  the  morning,  ice  appeared  in  the  watch-glasses  on  the 
grassplat  and  raised  board  ;  the  heat  of  the  air,  as  measured  by  a 
naked  thermometer,  being  then,  at  4  feet  above  the  ground,  39§°, 
and  at  7  feet,  40i°.  At  2h.  ice  was  observed  in  the  pans  in  the  pit, 
while  a  thermometer  in  the  air,  2£  feet  above  the  ground,  was  36^°. 
Shortly  afterwards,  ice  began  also  to  form  in  the  pans  upon  the 
grassplat.  The  temperature  of  grass  fully  exposed  to  the  sky,  was 
at  the  same  time  30°,  while  that  of  the  earth  an  inch  below  the 
bottom  of  the  grass  was  45°.  During  the  time  of  these  observations 
dew  formed  copiously. 


EXPERIMENT    II. 

My  next  attempt  was  in  the  manner  mentioned  by  Mr.  Williams. 

On  the  evening  of  the  22d  of  May,  I  encompassed  a  square  piece 
of  level  ground,  the  sides  of  which  were  3  feet  long,  with  a  border 
of  earth  4  inches  high,  and  filled  the  area  with  dry  straw.  On  this 
were  placed  several  of  the  earthen  pans,  which  had  been  formerly 
used,  and  a  few  smaller  vessels,  all  containing  unboiled  water.  After 
an  exposure  of  little  more  than  an  hour,  water  in  a  watch-glass 
upon  the  straw  was  found  frozen,  the  temperature  of  the  air,  2  feet 
above  the  straw,  being  then  37°.  In  half  an  hour  more,  ice  began 
to  appear  in  the  earthen  pans,  while  a  thermometer  5i  feet  above 
them,  this  being  the  height  at  which  Mr.  Williams  used  to  suspend 
his  instrument,  was  36°.  The  air  soon  after  became  colder  ;  but 
its  temperature  was  never  less  than  33°,  though  taken  by  a  naked 
thermometer,  which,  as  was  before  said,  upon  a  clear  and  calm 
night,  occasions  the  air  to  seem  about  2°  colder  than  it  really  is. 

It  might  be  inferred,  from  what  is  mentioned  by  Mr.  Williams, 
that  the  temperature  of  the  straw  beds,  on  which  the  ice-pans  were 
set  at  Benares,  was  always  found  by  him  above  the  freezing  point, 
for  this  reason,  that  the  straw,  from  containing  no  moisture,  could 
not,  like  the  water,  grow  cold  by  evaporation.  I  had,  therefore, 
been  surprised,  during  the  first  experiment,  for  I  had  then  but 
little  acquaintance  with  the  phenomena  of  cold  observed  with  dew, 
that  a  thermometer,  laid  upon  an  exposed  part,  of  the  straw,  was 
always  below  the  freezing  point,  after  ice  had  begun  to  form  in  the 
pans.  On  reading,  however,  his  account  of  the  process  a  second 
time,  with  increased  attention,  my  wonder  ceased.  For,  as  the 
pans  he  speaks  of  were  large.,  and  touched  one  another,  and  as  all  the 
pans  employed  in  India,  for  the  making  of  ice,  widen  as  they  rise 
from  the  bottom,  like  our  milk-pans,  the  thermometer  placed  by 
him  on  the  straw  must  have  been  secluded  from  all  view  of  the 


APPEARANCES  CONNECTED  WITH  DEW.  59 

sky,  and  would  therefore  mark  a  temperature  much  higher  than  if 
it  had  been  laid,  as  in  my  experiment,  upon  straw  fully  exposed  to 
the  heavens.  On  this,  the  second  night,  therefore,  I  placed  a 
thermometer  under  the  edge  of  one  of  the  pans  lying  on  the  straw 
bed,  and  found  it  some  time  afterwards  6°  higher  than  a  similar  in- 
strument upon  a  part  of  the  straw  bed  which  was  uncovered.  Gener- 
ally, however,  the  difference  was  not  so  great.  If  my  pans  had 
been  large,  like  those  of  Mr.  Williams,  I  should,  no  doubt,  have 
observed  more  considerable  differences  ;  for,  in  consequence  of  their 
smallness,  I  could  not  lay  a  thermometer  on  the  straw  bed,  so  as  to 
be  fully  screened  from  the  sky  by  the  edge  of  any  of  them,  with- 
out its  being  almost  in  contact  with  the  vessel,  every  part  of  which 
was  always  colder  than  the  sheltered  straw. 

Much  dew  formed  in  the  course  of  this  night.  The  greatest  dif- 
ference marked  by  me,  during  it,  between  the  temperatures  of  grass 
and  of  air,  was  6°,  and  between  those  of  air  and  a  fully  exposed  part 
of  the  straw  bed  9°. 


EXPERIMENT    III. 

This  was  begun  on  the  evening  of  the  16th  of  October,  and  was 
likewise  made  agreeably  to  the  method  related  by  Mr.  Williams. 

Ice  appeared  in  the  pans,  when  the  temperature  of  the  air,  at 
the  height  of  5i  feet,  was,  according  to  the  naked  thermometer,  37°. 

On  this  night,  I  placed  upon  the  straw  bed  a  dry  earthen  pan, 
among  those  which  contained  water,  and  found  the  inside  of  its 
bottom  to  be  as  much  colder  than  the  air  as  the  water  was  in  the 
other  pans,  before  ice  appeared  in  them.  After  the  water  had  begun 
to  freeze,  no  proper  comparison  could  be  made  between  its  tempe- 
rature and  that  of  the  empty  pan.  This  pan,  in  the  course  of  the 
night,  attracted  moisture,  which  was  afterwards  converted  into  a 
film  of  ice. 

But  the  chief  fact  established  by  the  present  experiment  was,  that 
water  may  freeze  at  night,  in  air  of  a  temperature  higher  than  32°, 
not  only  without  any  loss  of  weight  from  evaporation,  but  with  a 
gain  of  weight  from  an  opposite  process. 

I  had  observed  that  water,  exposed  early  in  the  evening  in  the 
open  air  to  the  sky,  lost  a  little  weight,  in  the  course  of  a  clear  night. 
This  I  imputed  to  evaporation  taking  place  before  the  water  had 
been  cooled  enough  to  condense  the  vapour  of  the  atmosphere,  and 
to  the  weight  gained  afterwards  being  insufficient  to  compensate 
the  previous  loss.  I  exposed,  therefore,  on  this  night,  water  to  the 
influence  of  the  sky,  until  it  was  cooled  to  34°.  Of  this  I  put  2 
ounces  into  each  of  two  china  saucers,  which  had  also  been  exposed 
to  the  air,  and  then  placed  the  saucers  upon  the  straw  bed.  In  the 
morning  a  thin  cake  of  ice  was  found  in  both  saucers,  one  of  which 
had  gained  2£,  and  the  other  3  grains,  in  weight.  Dew  was  also 


70  ESSAY  ON  DEW. 

copious  on  this  night.     At  one  time,  grass  was  9^°,  and  the  exposed 
part  of  the  straw  bed  12°,  colder  than  the  air.* 

It  must  be  evident  to  every  person,  that  the  formation  of  ice,  in 
the  three  preceding  experiments,  was  the  effect  of  a  natural  opera- 
tion, similar  to  that  by  which  the  same  substance  is  produced  in 
Bengal.  These  two  facts  must,  therefore,  have  a  common  cause, 
and  this  has  been  shown,  by  the  last  experiment,  independently  of 
what  was  said  before  in  this  Essay,  not  to  be  evaporation.  It  is  also 
clear,  that  the  cold  induced  on  the  water  in  those  experiments,  had 
a  common  cause  with  that  observed  at  the  same  time  upon  the  grass 
and  the  straw  ;  which  latter  cold  must,  in  consequence  of  proofs 
formerly  given,  be  admitted  to  have  arisen  from  the  radiation  of 
the  heat  of  those  substances  to  the  heavens.  A  necessary  inference, 
therefore  appears  to  be,  that  the  formation  of  ice,  in  Bengal,  in  the 
circumstances  described  by  Sir  R.  Barker  and  Mr.  Williams,  must 
likewise  be  attributed  in  by  far  the  greater  measure,  if  not  altogether 
to  a  loss  of  heat,  which  the  water  suffers  by  its  own  radiation,  while 
situated  in  such  a  manner,  that  it  can  receive  little  heat  from  other 
bodies,  either  by  radiation  or  conduction.! 


CONCLUSION. 

The  experiments  which  were  made  by  me  on  dew,  and  other 
subjects  treated  of  in  the  preceding  Essay  were  unavoidably  attend- 
ed with  many  inconveniences,  which  were  the  more  felt,  as  my 
health  had  long  been  feeble,  and  as  my  professional  duties  obliged 
me  often  to  return  to  London  in  the  morning,  without  having  pre- 
viously taken  rest,  after  the  whole  of  a  night  having  been  spent  in 
attending  to  the  objects  of  my  pursuit.  The  inconveniences  here 

*  The  greater  cold,  observed  in  this  and  the  preceding  experiment,  upon  straw 
than  upon  grass,  is  to  be  referred  to  the  shortness  of  the  latter,  by  reason  of  which 
heat  was  readily  communicated  to  its  upper  parts  by  the  earth. 

j-  On  the  evenings  preceding  the  nights  during  which  ice  is  produced  in  Ben- 
gal, the  temperature  of  the  water  exposed  in  the  pans  is,  probably,  often  60°  or 
more.  But  water  of  the  heat  of  60°,  if  exposed  in  a  shallow  earthen  vessel  to 
air  of  the  same  temperature,  during  the  day,  while  the  weather  is  calm  and  clear, 
will  loose  about  3°  of  heat  by  evaporation.  A  cold  from  this  cause  may,  there- 
fore, concur  with  that  from  radiation,  and,  consequently,  may,  in  Bengal,  ac- 
celerate somewhat  the  formation  of  ice.  The  influence,  however,  of  evaporation 
there,  in  this  respect,  should  the  state  of  the  air  with  regard  to  moisture  still  per- 
mit it,  which  must  often  not  be  the  case  while  dew  is  forming,  will,  as  the  night 
proceeds,  gradually  diminish,  and  at  length  almost  disappear,  before  the  freezing 
of  the  water  commences ;  since  I  have  lately  shown,  that  evaporation  from  water 
of  32°  produces  very  little  cold,  even  in  the  day-time.  Indeed,  it  seems  to  me 
much  more  probable,  that  on  a  clear  and  calm  night,  though  in  a  dry  winter  of 
Bengal,  water,  at  the  temperature  of  32°  will  acquire  warmth  from  the  formation 
of  dew  upon  it,  than  that  it  will  become  cold  from  evaporation. 


APPEARANCES  CONNECTED  WITH  DEW.  71 

alluded  to  were  indeed  so  great,  that  I  was  twice  or  thrice  obliged 
to  intermit  my  labours  for  several  months  together,  and  at  length 
found  it  necessary  to  cease  from  them  entirely,  before  I  had  nearly 
completed  the  plan  which  I  had  formed.  I  take  the  liberty  of 
mentioning  these  things,  to  excuse  in  part  the  imperfections  which 
will  be  observed  in  what  I  have  written,  as  some  of  them  would, 
no  doubt,  have  been  removed  by  a  further  interrogation  of  Nature. 


THE    END. 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

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JUL  1 9  1062 


LD  21A-50/n-3,'62 
rf!7OO7sl  0147fi"R 


General  Library 
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